Really it depends on what you are keeping your babies in. Generally the bigger the tank they are in, the faster they grow, assumming that the water quality is good. Ive never breed guppies but my platies it takes about 3 months or so before I feel comfortable in putting them in the main tank.
My female had a batch of fry about two weeks ago, I didn't separate them so some of them fell victim to natural selection, but I still have around 8-10 of them swimming freely around.
Generally speaking once the fry are bigger than the mouth of the fish in your main tank, it is a safe bet that they wont get eaten, probably about 2-3 weeks. My fry are a little smaller than the mouths of my other guppies and they are being ignored.
Congrats on your new additions, you must be such a proud mommy!!
I would say about 6 weeks should be sufficiant, but like NINILIE said, have lots of floating plants and hiding places, most of them should survive that way.
Good luck and keep us updated!!
One note to this ...it is my experience that if you leave the fry in with lots of cover they are generally ignored as stated above. You will loose the weaker and slower ones to predation....BUT....if you remove them and then add them back to the tank at a later date the adults WILL see them as a food source and will actively ( and happily) hunt them down and eat them....lol.