Depends who you ask. Some people say instant some say, 3 days...who the hell knows.
Put the strings back to pitch right away, its going to bend it back a bit (even more so with a bass). A Small adjustment will be needed then too probably, then just keep your eye on it over the next day or two.
Also, don't be afraid to make small adjustments with the strings at pitch (ya, I know some of you will go ape-**** on this but **** right off).
Usually my process is as follows (lets say there's a big ass bow in that sucker):
-Make major adjustments while strings are off/ slacked depending what I'm doing, get it to where it looks good
-Put strings on and up to pitch, adjust further. Careful to retune after each adjustment tho!
-play play play, check it after an hour...a few hours, then the next day.