They vary a lot. It depends on location of where it came from, the individual fish, and which species it is. Some large angels, such as the majestic or blueface, can go from tiny blue and white juvi to full adult in less than a year, however, on the other end, an angel like the koran or bluering can take YEARS. The Emperor is somewhere in between..about 2 years. Some will never change simply because they do not like their environment. (too small a tank, bad water quality, etc). I hope this helps, at least some. I'd expect it to start within a year and end in the next. I suppose it also depends on the age of the fish when you buy it. (generally, the smaller the fish, the younger, but not always)