3pm is the prevalent time that I've seen. I can understand a room not being ready and I will certainly wait if it's an upgraded room. Only once was I told I must wait until check-in time. That's by someone I had not seen at that property before, and he was still learning the ropes. A quick 'Hi Ron' with the manager (named Ron) freed up the room right then.

I would raise a ruckus and escalate with MR if it took more than a few hours to get me into a room. And I would expect an honest attempt to make me comfortable during that time. An open bar tab if no lounge available, or a comp'd dinner that night. Everyone will have their own pain threshold and should manage to that.