It depends on you. I've had my lip pierced for almost 3 years and I can't leave it out for more than an hour or it starts closing. My friend has had his pierced since late last year or early this year (so less than a year) and he can leave it out longer than I can. If you're just taking it out long enough to clean it, change it, etc., don't worry too much about it. By now, it's healed so it will take a while to actually close (I'd say a minimum of one hour, a maximum of several days). If you ever take it out and forget about it for a while, push your stud or hoop through from the outside, first, to make sure that the inside isn't closed (the inside closes first), then put it in the normal way. Been there, done that a few times lol.
at 9:53 PM on Aug. 27, 2010