I've been through this. Much of this was solved by my DS growing older & getting more mature. Important thing to remember those who makes these games don't make them easy to put down. And judging how much time an enjoyable activity takes requires experience and maturity. These games are addictive and often you can't just "hit SAVE" any random time. I more had this problem with RPG console games than Minecraft. But it was NEVER good to walk up to my son at that age and say your game time is over now. Many games require finishing an activity within it before you can find a save location.
I learned to make sure I gave my son warnings. Game time will be over in 10/15 minutes. Now you have 5 minutes, it's time to wrap things up and find a save location. You could also do this with an alarm clock, or clock tool on the computer/tablet. Being consistent and transparent helped. We also made a rule that if you thew a big fit (he was allowed to wallow in a normal tone of voice) games time might be taken away for a few days.
