I think I get about 4-6 weeks of nonstop play out of each league. Then after that I putter out. Going for 40/40 or a mirror or 100 (pre super breaches) would definitely give me more to do, but I don't really care about that stuff. I've been so close to 36/40 every league, but I don't even use MTX's so it means nothing to me personally.
What I've been doing now is basically just using my moderate amount of currency (10ex+) and just trying new builds I created. They're like, 30% successful. It's a good way to get better at the end game and min-maxing of the game. It's just kind of a testament to how good their game is that an essentially single player game, which I hate, is so good I can play it for hundreds of hours each league. And it's not even on the grind your dailies to get uber faction mechanic that all MMO's are. It's just genuinely fun to try things out with items you find or make.
GGG is the fucking tits.