That will depend on what size tank they're in, what the water quality is like as they grow, and how often and what kinds of food they are fed.
My molly fry are almost the size of adults at 13 - 15 wks. Mine get daily water changes, 3 - 4 times/day feedings, and are growing in fry tanks or the 90 gallon main tank. I keep about 10 - 15 fry in a 30 gallon tank, and run extra filtration. If waste gets to be too much, the nitrate levels rise and growth is stunted, internal damage is done, fish get sick, etc. In pristine conditions, look for the balloon belly to start showing about 6 - 8 wks.