I really don't know how fast it would be, you could try in the cichlid forum. Or there's an angelfish society out there. Our usual recommendation is not to be tempted because if the change-up somehow goes wrong, its more painful to have to give up the fish entirely. Maybe arobinson will happen along, he's good on sizing.
~~waterdrop~~
Thanks WD *gets out the ruler*
Agreee with waterdrop and oldman, we dont usually ever recommend putting fish into a tank that isnt big enough even with all the intentions of upgrading. If anything should happen which means you dont finally upgrade then the fish gets stuck in an inadequate tank.
As oldman said, with angels you're probably looking at around a year before it would definatelty require a taller tank, failure to do this would end up stunting the fishes growth. I'm not sure if you've heard the old myth 'fish only grow to the aize of the tank they are in' but that isn't 'strictly' true. Yes a fish will become stunted and its body will stop growing, the problem is that the fishes internal organs do not stop growing and end up outgrowing the body resulting in the fish growing deformed and experiencing a slow painful death, hence the reason we always recommend putting a fish in a tank that is big enough for the fishes entire life. The problem with stunting is that its very hard to tell at what point the fishes growth begins to become stunted so you could keep the angel in that tank for a year but then find the damage has already been done.
A well looked after angelfish should be expected the reach normally between 7-9" in height, they are a fairly lazy inactive fish but due to their height, they require space to move up and down. The height of the tank is much more important than the length of the tank for angelfish. As WD mentioned, the minimum for a fully grown angelfish would be 17/18" of water, this is not including gravel/sand etc although ideally angels should be in 2ft high tanks.
Andy