One of our guppys had 41 fry the first time. The 2nd time ( 3WEEKS LATER!!) she had 15 I think. They do breed very easily and they tend to still have a gravid spot. Our other guppy gave birth to about 30, still has gravid spot and her belly is growing, but at least we have an idea of when she is due again and the signs to look for.
As for how ong to keep them seperate. I have read many bits of info and it's seems to be when they are at least 1 inch long and can't fit in the big fish mouths. We tried one of our babies, she was big enough to go in the tank but the other fish chased her so we removed her straight away. They didn't catch her so no damage was done and she is still happily swimming around with her friends her own size. There's no rush. We have a seperate tank for the fry and they seem very happy.