Usually at least a week and up to 2 depending what it is (the bigger ones tends to fall off faster than the smaller crystals). Usually mine start falling off after a week or so because I starting picking at them (and because I never wear gloves when cleaning etc).
I do base cure, colour cure, colour place gems and lightly press down to get them embedded, cure, top coat - putting a bit extra around/over the gems and giving it a second to pool then cure.
I have used the CND glue too before (can't remember the name of it- it's a browny bronze tube) and those gems weren't going anywhere.