I mean, that's natural right? My girls help us test our games, and we always try to put an educational bent of some sort in all of our games whether it be pattern recognition games, basic economic games, or language games and puzzles. That's the point, though. The games that I allow my girls to play always have some component designed to teach them one lesson or another or to give them a creative outlet that I feel that they need. Ultimately, I can't answer this question for them and have them apply it to their children. If I've learned nothing else in my life, it's that children start being individuals very, very soon after they're born. Things that work for one child may be completely detrimental to another child.
Here's my very simple determination of how much video game time my child gets, and I also added my standards for my own kids.
1) How are his or her grades?
My kids have to have phenomenal grades for me to allow a lot of gaming. Thankfully, they do, and they understand that their grades tie directly into their privileges.
2) How much total time would she LIKE to spend on the computer?
The more my kids put aside other things (crafts, practice, going outside) to play computers, the less total time I give them without regulating it heavily. Basically, I am enforcing the balance. I also enforce breaks on a regular basis.
3) What does she want to play?
If she wants to play a game that I feel has education value over a game I feel is a brain-dead button masher, I'll give her far more time to explore the game. Also, if I feel the game has some value to teach my child, then she has more freedom to explore it.
4) How good is she at the game?
Once my child masters a game, I tend to start encouraging her to move on to another one. While there is much to be learned in repetition and dealing with patterns, the amount a person learns from a game he/she has mastered is much less than delving into something new. SOME games defy this. You can play them endlessly, and your brain would still be learning from it. (Check out the Tetris Effect.)
It's entirely possible that my answer will not help at all depending on the person your child is. This is a general guideline that works on MY children, but as I'm often asked for advice in this department, I figured I'd make a blog post about it. Hope it helps!