An action which is available "at will" has no limit to the number of times per day it can be used, save that it can only be used once per round. Whatever action or other option it is, it normally takes as long to use as it otherwise would, so a spell-like ability which corresponds to a spell with a casting time of "one action" still takes one standard action. Generally, I would say that "at will" abilities must be used on the character's turn who is entitled to the ability, so unless you have a monster with feather fall at will as a spell-like ability, you're stuck waiting for your turn.