However long it takes for them to get sharp. For kittens, they grow faster so maybe once or twice a week. Also depends upon how much they sharpen them. Just don't cut into the pink part (quick).
I try to clip kittens more often to get them used to it. maybe do a few a day just so having feet handled is more normal. Far as NEED to be done-it depends alot on how short you go and what they eat.