Too high is when the City comes and mows it for you, then sends you a bill for $100.
When my boys & I were doing a sideline mowing business, we usually cut the grass shorter in the spring (about 2 inches) and a little longer in the summer (about 3 inches). In the spring, the sun is your lawn's best friend. In the summer, it can become its worst enemy.
Honestly though, everyone has their own opinion on this one. I've seen people practically scalp their lawn all summer lawn, and it looked good. Others had it pretty long, and it looked good too.