It's 3/4th of a day park.
This is subjective, but with me, I usually get tired out after 5 hours and need a rest. So I leave the park, go back to my hotel, rest, and then go to another park for the evening.
Staying in a park for an entire day is too much, I don't know how people do it. That's why most of the time when I'm returning in the afternoon, I see many leaving at 5pm, tired.
It also depends on the park hours when you are visiting. If you go during an off peak time of the year, the park will close early at 7pm, so you'd have to stay there all day. If it's the summer time, and they close at 10pm, then you can leave, and come back after resting.