How long does stage 6 last?

None of the stages have a definite duration. In fact, a person in “stage 6” may revert back to a previous stage or even jump ahead to the next stage. Rather than stages, dementia is more of a continuum where someone can vacillate between stages. Typically Alzheimer's can last anywhere from 8 yrs to over 20. As already mentioned, your dad's doctor may be able to give you an decent estimate.

Do also know that every case of dementia is as unique to the person as a thumbprint. There can be no guessing, and your doctor is always the best person to "guess" at the prognosis. I recordment this month's AARP magazine article on Tony Bennett. His family has come forward about his dementia, and it is a fascinating article with many resources and much information. One of the thing mentioned is that the condition of the person struck with Alzheimer's before they get the condition, and the care DURING the course of illness does make a difference, but that there are many factors present that make each case unique.

https://www.dementiacarecentral.com/aboutdementia/facts/stages/#scales