Hmm, tough question. I'm assuming you mean D&D here. Even still, it's highly dependent on the type of campaign. Right now in my gaming career, I hate high-level "standard" D&D play. Combat is like pulling teeth. I swear the last few battles out 15-16th level group have fought have been riddled with oversights and errors. It's just a pain to keep track of all the modifiers.
So for standard D&D with a standard level of magic and items, I would say levels 8-12 I guess.
Edit: I voted for 10 for a typical campaign. Hmm, maybe I should have voted for 8. I don't know, I just started playing in a "lower" magic campaign at 1st level and it's a lot of fun.