WI state parks check in time is 3:00 PM, check out time is 3:00 PM. More than once I've had people wanting to get into "my" site at 10:00 am because they arrived early. Never once have I been hassled for it. They know the drill.
I've also arrived at 3:00 only to wait for the previous occupant to leave. Once we arrived at about 2:45 and the previous group was still there and just starting to make a meal over the fire. I got out and asked and they said they didn't know check out was 3:00. They took their time eating and packing, it was after 4:00 before we got it the site. They left the site a mess. Cigarette butts, and I mean at least a couple of hundred of them, and other garbage. I let the rangers know. That kind of stuff can get campers banned from being able to get sites.