She can become gravid again almost immediately even without mating since they store sperm. Gestation is usually around 60 days, depending on the molly species and exactly when she chooses to fertilise the eggs, but can be between 50-70 days.
You'll know that she's close to dropping them when she develops a sort of boxy, squared off look, that usually means they'll be birthed within the week. When she starts actively hiding in plants or behind things, or sits closer to the gravel a lot and is breathing hard, that usually means she's labouring, which can take a couple of days. Keep an eye out for other fish pestering her during labour;
other fish love a chance to get a free meal of some fresh fry. Lots of live plants to hide in is better for her than a breeder box or net, or moving her to another tank just before she gives birth.
If you want to save as many fry as possible or don't want her to give birth in the community tank, best to move her to her own tank early on, before she's too heavily gravid. Give her time to settle in and de-stress, since stress can affect the pregnancy and labour. Then once you're sure she's finished dropping her fry, can move her back to the main tank.
Some people use breeder boxes or nets. I dislike them for the mother since it can put a lot of stress on her to be put in one. But you can let her birth in the tank then catch the fry and move them to a breeder box. I don't bother since a large batch can only stay in the breeder box for a couple of weeks anyway, so since they need to go to a grow out tank or be released back into the main tank anyway, I raise some in the community, and some go straight to a grow out tank. As you can see here, plenty of fry with their mother, survive in a planted community tank
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And with the dad!