Does anyone have a great earning tablet time method that you use?

I just bought my kids Kindle Fire's, in the past we've only let them use their LeapPads on VERY long car rides, but the Fire's have so many great educational (and not) games I think I'd like to let them earn tablet time but I'm not sure what that should look like.

I've considered - earning minutes for chores, or making some kind of "caught you being great" chart.