We have limits on all video game time. The kids, who are homeschooled and have a lot of free time everyday, get 1 hour Monday through Thursday. After dinner on Friday, they can play whenever they want for the weekend. This comes with a few other stipulations also, such as chores and behavior. We also do a lot of family stuff on the weekends, which is not optional, so they get to refocus on real life.
My 12 yo ds and 9 yo dd LOVE minecraft. I got them each a shirt for their birthdays that says "ONE MORE BLOCK. ONE MORE BLOCK. ONE MORE BLOCK. ONE MORE BLOCK." My dd almost peed her pants she thought it was so funny! I think it's a great game. The kids are really proud of things they build, and have a ton of fun breaking them too!