How long do I have to leave in my belly button ring before I can change it?
Generally three months. The piercing itself can take quite a lot longer to heal.
Make sure you keep the piercing clean, apply antiseptic cream to it every day and try not to play with it too much. Remember a piercing is a wound and should be treated like one.
4 months :( Im shocked they didn't tell you when they pierced it, they usually make a big deal about it unless you did it on your own
3 months at the very least because it needs that time to heal up a bit