Do a search for the username "Gillybaby" - or go to his website:
http://www.gillybaby.com/index.php?option=com_content&task=view&id=76&Itemid=87
My emporer is changing pretty much at the exact same rate as his...... I reackon 12 months from start to finish whould get it to a point were you could call it a "full" adult.......... But it could take a couple of years to get the full, straight, perfect lines of a large adult....... mine has grown from abotu 3" to about 6.5" in about 10 months....... I am guessing that by the 12 month mark he'll be around 7" and have "adult" colours...... but by his current rate of growth, I would expect to see him a good 9" by the end of 2008....... end of 2009 I would hope he'll be around 12" plus..... and by then I would expect him to have a full and complete change.
Something to rembember is that alot of people keep these angels in reef tanks..... 180 reefs seem to be the most common...... and when you are dealing with a reef tank you are on the edge of a constant compromise between feeding the fish enough that he won't eat the corals, and not feeding him so much that the water quality slips...... and in my opinion that means that the typical emperor is nearly getting what he needs, but not just quite enough to go throguh a compelte and sucessful colour change........ the other scenario is that they are in FOWLR's, where water quality is not such a concern...... but that lesser water quality also impacts the fish......
I don't have that problem - mine is in a 240 reef, but its a reef where fish feeding is first priority..... and water qualtiy is dealth with secondary - but none-the-less is maintained at proper "reef" conditions...... so the fish gets the best of both worlds, and I think that is what is needed...... Gillybaby has pretty much the same philosophy........
So..... the short answer is, in my view, about 12 months under somewhat "approaching ideal" conditions....... and as you sip away from those conditions, be in water quality or diet then the longer it will take...... on a sliding scale if you will..... from about 12 months to never......
HTH
Matt