All night actually and yeah probably it is actually a supernatural effect so technically it does not even have a verbal component though it is debatable how quiet he can actually be given its description and it is the only way to keep some of his abilities up past 1 round.
Edit: I looked it up and it Pazo ruled that it is the same as a verbal component to a spell which means that he is laughing as softly as Tenshi allows verbal components to work at. What I find really interesting is that it is ruled as not an actual sonic effect and no one need hear him (works in a silent spell, underwater, ect) the attempt at honest laughter is required but actual sound isn't, make me want to figure out a way to get a silent spell on him that he can turn on and off.