Within a few weeks... BUT (lol I pun)...
before HRT I'd spent 35yrs powerlifting. So the butt and hips were sizable, but with muscle right up to the skin with no significant fat. As fat began to be stored there, sitting, laying down, got noticeably more comfortable, and I could feel the change in size when laying on my back or side.
I'm also eating at calorie balance, while scheduling meals and size of meals to provoke intervals of fat draw down and fat storage. eg, exercise on an empty stomach, eat only after muscles have recovered and somewhat restocked glycogen. Maximizes change in fat distribution without much change in weight.