Do dementia patients have periods of normalcy and if so how long can it last?

Since the OP hasn't returned to give us any more details it is hard to know what might be going on - it is certainly possible that a medical professional pulled out a dementia diagnosis based on nothing more than her age and a period of confusion brought on by an acute condition like a UTI.

From the OP's profile:

"Caregiver for mom-in-law has dementia, but now seems normal"

Errrrr... Shastagirl, do you mean that you are concerned about your 91 year old mother-in-law, and that it is your mother-in-law who has been diagnosed with dementia?

If that's correct, and you're now questioning the diagnosis, would you like to say a little more about her recent history? How the diagnosis was reached, what has changed recently and for how long her apparent 'normalcy' has lasted, that kind of thing.

When extended family talk to my Mom on the phone, they seem to think it is 'us' and that she is fine. She's not fine, and when you ask her questions, she will answer you pronto, but her answers are SO far from what is really going on. She can talk to family on the phone, minutes later, not even remember she has had a conversation with them. Read up on showtimers, maybe. They can seem so lucid and normal, but if you are with them every day you get that is not the case overall.

My dad has many periods of time he is lucid but it doesn't seem to take much anymore to throw him off the rails. About the time I think things are going well and I relax a bit, I get a phone call (he still lives on his own) and his anxiety has amped up, he doesn't sleep, etc. That lasts for a few days, he's back to normal for awhile, and then something else gets him going. I will say these periods are less frequent since he started on Seroquel.

My ex told me that his mom has mostly lucid times, but can suddenly go into "nonsense talk" or have memory dysfunction. So I think dementia begins as intermittent, & gets progressive.

Everyone’s different. I’m just giving you an example of how “normal” can come and go.

My Mom is pretty “normal” maybe 75% of the time, happy, singing, eating, able to converse although very little short term memory. (Aside from being blind and in a wheelchair). Then something will trigger a bout of “off the rails” agitation, insomnia, hallucinations. We think it’s caused by overstimulation, but who knows. After a day/night of that she konks out and will sleep for 24 hours, then be very tired for a couple of days, That pattern is the other 25% of her life, then back to “normal” for a few weeks. This week she was good Monday morning, then off the rails Monday night, couldn’t sleep agitated etc. and they kept her at the desk as she was trying to get out of bed. Tuesday she slept so soundly I couldn’t wake her up to eat. Wednesday she’s tired but getting back to normal. I will expect she’d be pretty good tomorrow, then ok for a few weeks. I have to try not to expect too much however, as things can change on a daily basis.

My dad has Lewy Body dementia, sometimes he is lucid for hours/days/weeks. Then other times hallucinations, delusions. Quite the roller coaster.

Yes, people with dementia can have periods of lucidity. They are not usually very long in duration, sometimes only a few minutes. But as MargaretMcKen said there is no determining how long they will last.

You can't pick what is normal, and you can't know how long it will last.