Like scotty said it depends on a lot of different things. Best thing to do is move them to a seperate tank and feed them often with a good quality food. Also regular water changes will help as well.
If you are able to, which I do, is as some of the fry will grow faster, take them out and move them to another tank that way you can seperate out the fast growers and more agressive eaters. That way the smaller less agressive fish can catch up in size.