prog is supposed to improve breast development (and often has some significant effects on libido and a few other things) and most people start it something like 6-12 months after starting E. that said, there isn’t much in the way of true medical research out there, so it’s hard to say with any certainty how effective it is and when the optimal starting time is (a lot of doctors will take the lack of research and use it to claim that prog definitely isn’t necessary, which is a lie).

in my experience a lot of girls try it for a few months and may or may not keep taking it depending on whether or not they personally think it made a difference (and whether or not they like the extra levels of horny). but it’s very much a “your mileage may vary, try it and see!” sort of thing.