I noticed a difference in my first month on T, but I wasn't crazy about my downstairs so it wasn't something I looked at with any kind of regularity. I'm sure it occurred over time in that first month and not all at once. I did find that it was more physiologically obvious to me than obvious by looking at it, at first. Like I could feel there was a difference but looking at it I didn't see much difference at first.
It's also been my experience that it occurs in spurts. So I had some growth in the first month, some more between 3-6 months, and some more around 9 months. Most statistics on bottom growth say that it maxes out around the 2 year mark.