I think 4-6 seasons is the sweet spot. Most shows seem to run out of ideas or get repetitive and feel like they're trying to milk it for all its worth if they go much further than that. Less than that and probably got cancelled too early, or there just wasn't enough story there to begin with. But also of course the number of episodes per season factors in to this a little too - Most network shows in the past were 20-24 episodes per season, but a lot of cable/netflix stuff these days is 8-12 episodes per season.