In my experience with mollies it varied a bit on the size/age of the fish. If yours is younger I'd say she could go any time. If she's acting more agressive that could be a sign, with my fish they'd usually start hiding a lot but I had lots of plants and cover which I hope you have or else the babies may become snacks. Otherwise you could put them in breeder nets/boxes but I find the clear boxes to be stressful for mom, so I'd catch as many as I could and put them in the boxes until they were big enough to swim with the others.
Hope that helps a little! Maybe someone with guppy experience can chime in
The tank is packed with plants, as in I can't find my fish sometimes haha. I hope when the fry are born I can find them because I do intend to raise them in a separate tank. I have a grow out tank ready, it has little Cobra in it right now as she is still not big enough to join the ladies. I have a mesh breeder. Assuming I can find the fry they will go in there for a week or 2 and then be transferred to the grow out tank. Any females will be kept to add back to the ladies' tank and hopefully there aren't any males haha.
Ah good :) if you have plants near the surface, check there when she has them. Also, don't know about platies, and don't know how often it happens, but I bought a young "female" Dalmatian Molly that turned male lol and it went after allllll my girls, and sometimes boys too >.> little gender confused fish haha, but good luck! Post baby pics if you can ^_^
With the platies its kind of hard. I purposely tried to get fish that were not pregnant lol. The platies don't always have a gravid spot like guppies do. Mine did not look pregnant at all. and then 3 days after I got her home, it looked like she swallowed another fish lol. Now a full week and 3 days later she looks like a balloon.