Add them as soon as anything in the new tank hasnt a large enough mouth to eat them really-with endlers and mollies i would say a month but as i say-depends whats in your tank?
I tend to wait as long as possible to add fry back now....I tryed adding babies back to the big tank that I thought there was no way theyd get eaten...nope they got ripped to bits first to make them small enough to swallow...