When I was raising feeder guppy fry (trying to make them big and fat) I fed four times a day, changed the water every day, and fed a different meaty (brine shrimp, bloodworms, earthworm, etc.) food at each feeding in adequate portions. The feeders were raised in a 10g at roughly 80 degrees F. The tank had java moss, java fern, and an air bubbler. They reached a useable size in about three months, but I believe the time it takes for guppies to be considered at adult size is something like four to six months. I could be wrong, though, but my guppies got nice and big with that procedure. It also matters where the parents are coming from, because if you have mutt guppies then they are likely to have issues with genetic stunting or other health problems. If you're looking to sell the fish you'll have to start watching for culls.
I'm raising another group of about 25 now in a 5g that's a little less strict (only 2x a day feedings, it's at room temperature, and there are other animals in with them) that are at a reasonable size at the month and a half point, but nowhere near as big as their mother. As soon as you start seeing secondary sexual characteristics I would suggest separating the males and females (this happens much earlier than you might think).