I have 2 10 gallons with only guppies. I don't keep the female in the trap, but I keep my eye on her as she gets kind of sqaure shaped in the belly. When she looks squarish, I scan the tank very carefully each day a few times.
Once the babies are born I catch as many as I can in a net and put them in the trap. They need lots of food, so you have to keep the bottom of the breeding net clean, but it isn't hard.
They grow fast, so they don't have to stay in there very long. Your babies need to be big enough not to get swallowed...as time goes on I have found that they need less time than I thought before you can put them in the tank free. I haven't lost many yet doing things this way. Only about 4 out of 45?
Anyway, if you save them all, you soon realize that you best have a zillion tanks to house the babies you see each month!
Once I let the babies out, I keep the adults well fed for a bit so they aren't as tireless chasing the babes.