If these are new rats (as in you brought them in from another location), they might need a couple weeks to settle in before they'll start to reproduce. Also if they are younger, they might need some time (a few weeks to a few months, depending on how old they actually are) to grow up a bit and get the idea of what they are supposed to be doing.

All else aside... once they do start breeding, gestation is about 21 days, give or take a couple.