I normally spam the hell out of a champ, then switch after like 30 games to a completely new champ when I get bored.. I guess that's why my champ pool is a bit fucked.
I also think its important to have a primary role, you may want to join a team one day for online tournaments and such, having 1 role mastered is a good thing but it doesn't mean you shouldn't play other roles also
PS: With team-builder in the works sticking with 1 role might be a lot more viable of a option
Champions that require a higher mechanical skill to master generally is very hard to get bored with, due to the fact that you can never fully "master" them (or just takes forever!) I find myself playing hundreds upon hundreds of games with vayne/draven, and just alternate between the two. Never gets old for me :0
I've never mained a champion. I don't think it's really that big of a deal I think it's more important to be able to play every champion well because in ranked your best champ wont get banned and you will do better playing a counter champion. That's just my opinion, maining is way to boring for me the only champions I can play for more than 5 games in a row are zed and kha really