I wasn't thinking about starting until I was a year into my estradiol only treatment or when and if I reach later Tanner stages of breast development.  At this rate, that will be much longer than a year.  Both of my doctors were ambivalent about progesterone.  Will prescribe it if I want,  but aren't sold either way on effects yet.  Not surprising,  given there's not a huge n out there with a controlled study.  My year+ timeline was just based on the fact that cisgirls typically have their progesterone kick in late in puberty.