You mean showing up with no ADR?
Odds are against you being seated without an ADR no matter what time you show up. How close to opening has no bearing on it. Disney schedules based on ADR in many cases. There are a few exceptions that always take walk ups but most will not take them, even if there is an ADR on MDE for that location and time. It's just the way they are set up.
You should open the app and book an ADR
As to how late they'll book ADR, right up till closing. If a park closes at 9:00 the last ADR of the night at that parks restaurants are 9:00.
And, it's unlikely that park hours will remain unchanged. They've been changing them just 2, 3 weeks out for more than a year. Don't assume the schedule you see now will be the schedule when your trip comes up
I ate at Le Cellier for lunch last week. We had a 12:15pm ADR and were seated closer to 12:30. The place wasn't very busy. While we were waiting to be seated, a few people walked in and told them to try back like 1-1:30pm (after the lunch rush). I think depending on the restaurant and time you're trying to eat, you could see if you could have some walk up luck?
No matter what restaurant you're visiting, you'll always want a reservation if one is available. If they don't have the staff, then they simply won't seat you, even if every table in the restaurant is empty. With a reservation, they'll seat you. When you search on DisneyWorld.com, most or all restaurants should show what hours they're open for a given date.
we have the last ADR at Coral Reef every trip - intentionally - 8:55pm. When we are seated the restaurant is full - by the time we leave we are the last one. we LOVE walking out of the park by ourselves - or should I say with our escort now that we leave via the IG. Oh wait that begs a question I'll have to find out - we're staying at RR and using the death buckets - do they run 3 hours after the park closes like the buses? Have to find that out!
We have also had a late seating that probably turned into the last seating because BOG is always behind - and walked out by ourselves at MK. It's a wonderful experience.
But to your question - even so we were one of two or three parties sat at the time - the restaurant was full. Can you try? Sure!