it depends entirely on the games. id imagine your nephew will start by playing ds games, which is fine, but god help you if you let him play CoD or similarly addictively easy to get into games. i personally believe you ready to play whenever soul-crushingly repeated defeat only drives you to win even more, because i only got into gaming after constantly dying in ultima underworld just made me want to finish more.
as for when to stop...doesnt parenting kinda take ALL your previous central interests away? id say stop the same time you think you should stop watching movies or listening to music, which is probably never, but if you dont have the time for games, its not a matter of you SHOULD stop, its matter of just not having the time to continue.