Basically, you've got about six months to a year before your navel will be healed. Your piercer should have talked to you about this, and/or you should have asked them. The skin heals from the surface in towards the center, so there's no tried and true way of telling when you're healed. Stay out of public pools and hot tubs for the next six months, at the very least, as the chemicals used in them can burn the unhealed body tissues and the open wound leaves you (literally) open to greater chances of infection.
There's an article on navel piercing in the body art piercing archive if you need more info
http://www.bellaonline.com/articles/art8538.asp
