My son has had a Gameboy since he was five and the Nintendo DS for several years, and I have never felt the need to limit his time on them except when he gets a brand new game that his friends are also getting and then he is competing with them to get to a higher level. He wants to make sure he is at a higher level than they are. He loves it when they call and ask him for advice.
My 10 year old son and his 59 year old dad, whose fun loving playful attitude keeps him young, enjoy playing history related role playing games and strategy games. I have to let them know about 30 minutes in advance when dinner will be ready so they can finish whatever level they are on. As long as they are able to get to a stopping point or save point there is no problem. It is when I try to tell my son that he has to get off a game immediately and he doesn't want to lose all his progress for that level that he might get a little unhappy about it.
I really like the leveling up thing, especially when my son applies it to his SAT prep game, math games, vocabulary games, and puzzle games.
But I would definitely limit his time on the Nintendo DS if I felt it was becoming a problem.
