Considering that HRT can result in quite satisfactory results over time, I think this is a rather sticky topic. Who gets to decide what is satisfactory, and how long should the woman wait before the HRT-induced results are deemed "final"?
If decisions are made case by case, you can bet that the bottom line will kick in and some women will be left with results that health care providers consider ample but the woman in question, and perhaps many other people, consider to be quite inadequate.
I've heard a few stories, perhaps apocryphal, about trans women who got breast augmentation surgery almost immediately and then made unexpected size gains on HRT. I figured that they were jumping the gun by getting surgery so quickly, but as long as they paid for it themselves...
For the same reason that trans men are advised to be on HRT a few years before they get meta, I don't think it's reasonable to expect insurance or national health care to cover breast augmentation when a woman has just started HRT. But I can't really say how long she should have to wait.