It's really highly variable from tank to tank and for different types of Acros(I assume you are talking about Acros, most other "sps" would be the same). When the coral feels "comfortable" is usually when polyps come out IME and that's a good sign the coral is starting to settle in to it's new environment. There are a few tanks out there that never seem to have good polyp extension and the corals seem to do fine healthwise. In my tanks some corals that haven't been stressed too severely will extend their polyps within a day or so, most within minutes or hours.
hth, Chris