What do you mean by "after"? If you mean within 3 months of getting the shot, then it is nearly impossible to get pregnant as that is what the shot is designed to do.

If you mean after that three month period has expired, then it usually takes a woman up to a year for her normal menstrual cycle to begin, and then she can track her fertility clearly and become pregnant.

It all depends on your body. I was on the shot for about 2 years and after I stopped and switched to a pill I got pregnant with in 2 months. It just depends! But as Eiri said they say it can take up to a year.