I think anything beyond 10-15 minutes is an unreasonable time to wait without being acknowledged (after being seated) or served (after ordering).

I have gotten up and left. I also once reseated my family in another part of a dining room after waiting too long for someone to take an order (I really wanted to eat there). When the new server greeted us, I asked to see the manager so I could explain. We were comped appetizers for our inconvenience. I never did find out why we were ignored, but assumed it was confusion among servers over whose table it was.