The time it takes to reach sexual maturity will depend greatly (and I mean greatly!) on the temp, amount of food available, quality of food available, quality of water, and duration of lighting.
I raised some baby sailfin mollies from a wild gravid female I caught in a local creek. They were put in a 20gallon saltwater refugium that had a ton of little bite-size invertebrates living in it. It was lighted 24/7 and temp was low 80's. Within a month and a half, I had pregnant females and more babies. There was plenty of live food, lots of light (ie, they never rested, just ate all day!), superclean water, etc.... Not to mention being strong stock b/c the parents were 100% wild caught.
You don't need such a "super" setup to raise the babies, though. Just expect at least a month and a half, maybe two months, before you can get a real identifiable sexual dimorphism going on.