How long should you leave a tongue piercing in after you've had it pierced?

What do you mean? You can never take it out, or it will close up! Every person is different, so some people might (after having it for a very long time) be able to take it out for a short amount of time (a few hours) and still have the hole, but other people will find that it closes up in less than an hour (even if they've had it for years). The cells in your mouth re-grow very quickly... Even if it didn't completely close, it might partially close, and then you'd do damage when you put the jewelry back in.

A tongue stud is not an ear-lobe piercing: You don't decide, each day, if you want to wear it or not! Consider it permanent.