Gestation for guppies/swords/mollies are about 30 days. Its hard to say exactly when because mollies/swords/guppies can store sperm and have multiple batches of fry without any males in the tank. This makes it hard to keep the population under control.
Be careful you do not over stock your tank. Also, as the fry get older you will have to start culling them when they start to have babies. You will get some fry with bent tails or bodies (a result from inbreeding) and you will need to cull them. I had a few guppy fry that I noticed this with. You can tell around the 4 week mark if they are normal or disfigured.
