What is an acceptable bedtime for elderly parent?

From my experience with my mother, she benefits and does best with structure, routine, habit. It helps her feel somewhat safe in knowing what is coming up next. So, I would try to establish a regular schedule of rising, and sleeping.

If your dad is ok with this and he sleeps through the night it's a great situation for you and your spouse!

Yes. My mom is 81 and also has dementia, and some days she's asking before dinner if it's bedtime yet. She dozes off a lot during the day (thank you, God), and still has trouble staying up until her 9pm bedtime. She usually gets up around 7am on her own, and if it's way earlier than that, I put her back to bed. Some days she surprises me and stays in bed until 9am. I'd always heard that the elderly slept less, but apparently the dementia makes a difference in their sleep requirements. It could be boredom in my mom's case. She used to be an avid reader, but now all she does with books is tear the pages out of them, so I put them away. Can't get her interested in TV anymore either.