My dad has dementia, when is it time to put him into a nursing home?

When he can no longer manage on his own is the time. It may come to a crisis point with a fall and hospitalization. DO NOT bring him into your home.
Depending on funding, he either qualifies for the self-pay of AL or for Medicaid for a NH.
Just pls, do not rescue him from advancing age and dementia by bringing him into your home or moving in with him.
Make sure the paperwork (POA, etc) is in order and with questions regarding your sib, try to keep her out of it (shared DPOA rarely works well). Contact his attorney and report that it's time or close to it, seek advice. You can consult with the local aging services.
Best to you and and yours in all of this.

Here's my opinion on this type of situation.

The one person that is carrying the load gets to decide when they need/want/have to set it down, period!

If your sister that checks on him and helps him is willing to continue, well, let her. Help her find additional help that gets paid for by dad to ensure his safety.

You can't just put someone in a nursing home. They must require skilled nursing care to qualify for a bed. So you all may be arguing over a non-issue.

Call the local to dad, counsel on aging and get him a needs assessment. This will tell you all what a professional believes he needs to be safe and taken care of.

There are options between home and nursing home, do the research and present the options. As for dad agreeing, well, have you ever heard a child that is asked if they want a spanking say yes? Never gonna hear a senior asked if they want to go to a NH say yes either.

Which one of you holds his DPOAs? That is the one that has ALL the power when dad can't make his own decisions.

Melann73, welcome to the forum. As to when your Dad would be ready for skilled care, that would be up to the Memory Care or Skilled Nursing Home to evaluate your Dad to see if he needs such care at that time.

Could your Dad budget for Memory Care? Memory Care is part of Assisted Living in a senior facility. Usually each resident has their own studio type apartment [or as my Dad would call it his college dorm room], and there is a monthly rent charge. The Staff in Memory Care are quite familiar with all the different types of quirks that come with Alzheimer's/Dementia. Dad was quite happy being around people of his own age group :)

My Mom was placed in a Skilled Nursing Home after she had a bad fall, which accelerated her very mild Dementia into late stage Dementia. She needed a village to help her as she forgot how to do so many different things and she was bedridden.

When he’s no longer safe living alone, it’s time. If he’s prone to wandering it’s time. If there’s a chance he’d inadvertently start a fire, it’s time. If he can’t adequately care for his own daily needs and no one is consistently and near constantly available, it’s time. Please know that asking him his opinion on this is unreliable at best. This is the time when decisions are made for him in his best interests. And when it’s time, know that it’s okay. We all do the best we can when given a sad set of circumstances. I wish you both peace

They always say they don’t want to, but when they start doing things that would harm themselves or others, it’s time to consider a different living arrangement. His questionable decisions could lead to his losing all his money in a scam. Or trusting someone to enter his house who could do him harm. He may start wandering. You don’t say if he’s sick other than in his mind, so it may be memory care, not a nursing home, that he needs. Please start researching that for him. Get evaluations from his doctor. At this point, he may no longer be capable of making sound decisions—ever. So you need to be the adults and take charge.

I have also asked my sister about in home care but she just wants to put him into a nursing home. This is really worsening the divide that is already between all of us