They start right away, for sure. I've got 8 Black Mollies I got for 25 cents each as fry a few months ago, rescuing them from the LFS.
One of the females swelled up noticeably, and before I could isolate her....*POW*...one (just one) Molly offspring last week. The mother is still so small at this age that she's not big enough for a larger brood.
She's already starting to get a "pooch belly" again. It's easiest to tell when looking at them head-on, as a gravid Molly (or guppy, Platy, etc.) will get noticeably wider, too.
v/r, N-A