I think over a year is probably fine. I started at 8 months and don't regret it, my boobs seem to be continuing to grow. There are people who report starting progesterone from day one and have normal breast growth.

You might already know this, but every time progesterone comes up I like to gently remind people that progesterone should be administered rectally to be effective, oral progesterone is filtered almost entirely by the liver.

It might be fine for cis women who produce endogenous progesterone and the less than 10% of the oral dose that actually reaches their blood is still helpful, but for transfem individuals, that won't meaningfully raise your blood progesterone levels to anything close to cis female levels.

At the advice of my doctor, I started progesterone at a little over 1 year of HRT, and I'm currently at 1.5 years, and in my mid 30s. They told me that the evidence of its effects are anecdotal at this point, which is why you get such unclear guidance for now. Progesterone in trans women hasn"t been nearly as well studied as estrogen at this point.

9 months to a year of E as a minimum is the advice I've encountered. 1.5 years is enough. I think cis women start producing it about 2 years after estrogen. You can start now or wait either is fine.

If you're worried about stunting breast growth, I have to ask, are your breasts currently growing? Mine seemed to have stopped growing at 6-9 months, and have been the same size since. Since starting progesterone they've gotten a bit rounder I guess, but still about the same size overall.

I've heard progesterone helps the mood of some, and causes depression in others. But for me, I don't know. It doesn't feel like anything. The changes, if they exist, are small or subtle. Maybe I'm taking the pills wrong.

most people ive talked to started prog at like 6 months to a year, just anecdotal tho. i plan on starting at 6 months

Starting around 6 mo would be ideal, but there's never a too late. The sooner within that period, the handier!