What is the longest time someone has been a caretaker? How long can it go on?

You are getting burned out as is common when one does all or the majority of the caregiving, and things must change if you're going to survive this journey with your husband.
You don't mention it in your post, but in your profile you say that your husband has Alzheimer's/dementia. Depending on what kind of dementia he has could determine how long it could go on. One of the gentlemen in my caregivers support group has been caring for his wife with Alzheimer's for going on 19 years now. 5 1/2 of those years she has been in a memory care facility, and has been in the final stage for quite some time now.
And my husband who was diagnosed with vascular dementia in July 2018, died in Sept. 2020 from it. However my husband had had a massive stroke in 1996, so technically I cared for him for 24 1/2 years. So you can see the time range can vary from person to person and from the different dementias, so it's very important that you're taking care of yourself so you can be there for him in whatever way that looks like.
Does your city have any Adult Day Care Centers? I would start there and see if you can bring him there at least 2 days a week if not more, so you can get a break and get done what you need to, and even take time to do some fun things you enjoy. You can leave him there up to 8 hours at a time, and they feed him and provide entertainment of varied kinds.
I would also "Google" to see if your area has any caregiver support groups either in person or on Zoom. They were a Godsend for me while I was caring for my husband and I still attend to now try and help others in the throes of things.
The fact that you're now screaming at him should let you know that something has to change as that is a sign of burnout. You MUST either bring in some outside help, or start looking into placing him in the appropriate facility.
And instead of screaming at him perhaps try going outside on your back porch or patio and screaming instead. That will allow you to blow off some steam while not taking it out on your husband who can't help himself.
It's hard this I know. I hollered at my husband several times, and I know that everyone who has cared for a loved one for any length of time has done so too(if they're honest)so don't beat yourself up over it, just start taking the baby steps needed to change the way you're doing things as you can't keep on like this for much longer, or it will be you who is needing to be cared for or dead, as caregivers have a 63% higher mortality rate than non caregivers.
If your husband is a Veteran they offer some assistance too.
I hope that you can get things figured out sooner than later and that you will be able to take some time to do the things that bring you joy.
God bless you.

Oh and by the way there's a big difference between a caretaker and and a caregiver. I'm just saying.

I’m already burned out after 2yrs. This is the hardest job I’ve ever had. I love my mom but I’m starting to yell too and that’s not me. I put her in daycare 3 days a week. She loves it! 24/7 isn’t easy for anyone. I have to shadow her whenever she’s up. Very unstable. I second guess myself all the time. We just retired and want a break. Looking into respite care for a month. Burn out is real!

"How long can this go on?"

Alzheimer's Disease has a life expectancy anywhere from 4 to 20 yrs. Other health conditions will of course effect this.

Most dementia types are progressive but will differ in severity & speed through various stages.

One caregiver can only go so far. Then a bigger team is needed. Either provided in home or by a team in a care home.

The time is coming where you reassess how you live. How you will obtain more care for him, whether your current home will still be suitable, if not, where he would go, then whether you would stay or move.

Before all those bigger decisions, first, how to get some help NOW. Can a day-aide be hired, 1-2 afternoons a week? For regular time off. Could he move into care for 'respite care' for 2 weeks? Give you a one-off break.

So many want to avoid getting 'strangers' in or ever considering a 'home'. I get it. Involving others is a hard step. But what's the alternative? You wear out 😞

If you don't wish to change what you are doing right now, at least consider an emergency plan in case you get ill.

Husband has been diagnosed for 16 yrs? That is a long time, I think, for ALZ or any Dementia.

For you, may be time to place him. You are definitely burned out.

First, you need to see an Elder lawyer about having you assets split. There are some retirement funds that Medicaid cannot touch. Your husbands split of the assets will go towards his care in a NH when they are gone, you apply for Medicaid. You then become the Community Spouse. You will remain in the home and be allowed one car. Your monthly income will be looked at and you will get some or all of it depending on what you need to live on.

Once you get that all worked out, you can place your husband. It will be easier to find a place if you pay privately, even for a few months. I started Moms application in April. She had 20k. I placed her May 1st and she paid May and June. My State gives u 90 days from application to spend down, get info needed and find a place for the recipient. In June I confirmed Medicaid had all info needed and July 1st Moms Medicaid started.

I helped my husband care for his mother for over 9 years. She had alzheimers/dementia. I quit my job, relocated back to his home state because the nursing home said we needed to help care for his mom. Now, I'm taking care of him since his stroke in 2017, diagnosis of vascular dementia in 2021. It's just him and me. We tried to get the VA caregiver stipend but were told "he's not bad enough." Will be appealing soon.

I know how you feel, believe me! I do pretty much whatever needs to be done around here. We can't afford to pay for lawn care, so I do it. I do the driving, shopping, etc. There are days I get "snappy" with him because of one thing or another. He will not accept home health coming in and staying with him while I go out. Respite care? No, he wouldn't go for being left somewhere, or even someone staying here while I go away for a few days. Not at this point anyway. So I go to the garden, I get away for maybe an hour or so running errands. That's my "respite."

Well, I took care of my husband with vascular dementia for 6 years. The first 2 years weren't too bad because I could go out for an hour or so without fear, but that changed when I found him walking along the road near our house, and he didn't know where he was. I spent the last 4 years taking him everywhere I went. That worked as long as he was cooperative, but when he was not, I pressured, cajoled, begged, promised him something, and sometimes screamed at him. Yes, I know that sounds bad, but those occasions when I screamed, it was because I had come to the end of my ideas, and/or patience. But I felt miserable when I lost it and apologized when it was over. When he would get angry at what things he could not do any longer, he took it out on me, and he accused me of all sorts of awful things. I did not deal well with that and often lost it then. That is when I looked very long and hard to find someone who would come in and give me some help on Care.com. Luckily, I found a retired RN with some experience with dementia patients. She was a God-send. I looked for the things I could do with the time and she was a very reasonable cost, It made a difference!! I could do errands without having to hurry, and without having to take him with me. I went to choir practice and felt joy again :) At first he didn't like it, but it didn't take long until he began to like her visits. I don't think I could have made it through without her help. My only regret is that I didn't find her earlier. He died 6 weeks ago, and I wish I could go back and remind him that I loved him, Don't be so hard on yourself, but do get some help.

My husband was diagnosed with Parkinsons when he was 52 and I was 50. We had about 7 years before everything went wrong. I was full time caregiver with conditions getting worse for 18 years. MY poor husband finally intentionaly stopped eating and drinking and passed 1 1/2 years ago. Degenerative diseases kill 2 people. I suffered a stroke 3 months after he passed. Get any support you can, church, group or private support. If your person can get hospice, do it! For 2 years they took him for 5 days every month! You have to put your needs first or you become ill too. I was looking forward to having a few years , I was a very youthful 77 , stroke ended that. I refused to place him in a nursing home. Maybe I should have.

I am on year 10 and mom is still going strong. Physically she was healthy when diagnosed with dementia. Mom has longevity in her family so I don't see an end in sight.

Have you looked into what he would qualify for with Medicaid? Daycare? In home care so you can get away for several hours a day? Long term care placement? I would make an appointment just to see what is available.

I am in my 7th year of caregiving. I have 42 hours a week of help from aides hired through my long term care insurance. Still, I am "fried ". My husband has a neurological disease and I must do everything for him, or the aide does. He cannot move without help. He cannot eat without help. He cannot get dressed or shower or sit up without help. We go nowhere. I have not been out of my local community in three years (pandemic did not help matters). I am now looking into counseling. I need someone to listen to me vent! This is the most difficult journey imaginable, especially when I realize that the only way I will be free is when he dies. Breaks my heart on a daily basis.

I'm 16 years into caring for my 95 yo mother.