I started wearing women's clothes as soon as I came out to my wife, long before I started HRT. Underwear full-time, of course, and skirts and women's T-shirts around the house. When I started dressing for the weekly support group meetings, I got a bra (to stuff, of course), skirts, tops, shoes, etc. Wardrobe acquisition was a slow but steady process.
I continued this pattern after I started HRT, until my breasts became too painful not to wear a bra. I wore sports bras under baggy men's shirts when I was in drag for comfort and to minimize them.
All that time, my wife and I were hanging out in second-hand and discount clothing stores. The deal was that, since I had to get an entire new wardrobe, it was only fair that she should get new clothes too. It's more expensive that way, but much more harmonious! We both got some very nice threads at very reasonable prices.
After I started HRT, I started paring down my male clothes to make room in the closet for my female clothes. Since I knew I would be full-time by mid spring, I got rid of all my summer men's clothes, and most of my cold-weather clothes as the weather got warmer.
I went full-time exactly three months after starting HRT. At that point, I switched from compression-type sports bras to padded bras.