We stayed two nights. We had time to just walk along the walkway and view the falls, do the spanish Aero car, the maid of the mist boat ride, the Journey behind the falls, the White Water Walk, Niagara's Fury...and spend an afternoon relaxing in our hotels pool. We did Niagara at the end of a 10 day road trip of Canada and by then we were just beat and needed an afternoon of *NOT* touring around!
If we had stayed more I would have ventured out to Niagara on the lake, which I've heard is wonderful.