An adult platy will eat fry at any point if they are crowded or hungry. Expect up to 30+ fry every 30 days from each female that has been exposed to a male, and this can continue for up to 4 - 5 months even after a male is removed. How big of a tank is this? When breeding platy, mollys, swordtails, guppys, its important to work with a very large tank. Platys can reach 5 inches full grown, and fry can reach adult size ranges within 13 - 15 wks if they are healthy. Its very easy to overpopulate any tank with these fish, which leads to water quality problems that will kill them all eventually.
I breed mollys and at the moment have 5 females in a 90 gallon tank with 2 males. Inside of 6 wks I must move most of the fry to other tanks to make room for new ones. (I typically move 60 - 100+ fry every 6 wks or less) Lots and lots of water changes will be needed in any size tank for breeding these fish. If you don't have a place arranged to take your fry, you may want to consider not trying so hard to save them. I supply a pet store with mollys every 3 - 6 wks, so I have a system of rotation, but it requires about 8 tanks for just my mollys. I have seen a lot of people wipe out their main tanks because they don't plan for the population explosions that are sure to happen anytime a female fish has been exposed to a male.