Most of literature says Livebearers in general (Guppies, Mollies, Swordtails, Platies, etc) take 3-4 weeks to drop fry from the point of impregnation. The higher the temperature the shorter the gestation period. If you dont witness the actual mating act thats OK because the females of the listed species all store sperm from previous encounters and will have several sucessive broods. Good Luck!