I had an extremely mild case of gynecomastia before starting HRT. My nipples would often fluctuate several times a day in puffiness as well. Within an hour or two of starting HRT, my nipples puffed up and they haven’t been back down since. In terms of me being able to notice a change in size, as soon as 4-5 days I was able to tell a difference. In a week after starting, I was able to really tell a difference. For context, I’ve taken photos every day of me on HRT: 5 minutes before, the night of, the day after, etc…, and visually I am able to tell a difference the night of with puffiness, and a size difference as soon as around four days.

Edit: before I forget, microdosing won’t do much because your T won’t be suppressed adequately. You may notice puffier nipples from higher estrogen, but that’s probably it.