For me it usually goes in cycles. There are times when I'll spend half my day on SM and others when something more important is going on in my life and SM takes a back seat. The good thing is you don't really get penalised for not logging in for a few days, you might forget to change your team for a cup game or whatever but your team's still there. Like Betty said I'm way too invested to quit just yet though, not until I lead PSV to that Division 1 title! Suppose it's different for the likes of Lawnmo & RK7 though, they've already been there and won whatever there is to win with their teams.
The other thing is that this league has become a lot more competitive over the past year or so. We've had some really good new managers joining....Betty at Inter, Alex at Bayern, even my old chum Klippity. All of them have been active and really added to the world. It's made things a lot more intense than they used to be though, and a lot harder to slack off!