DEPO shot help

I've read that when you get the DEPO shot after those 12 weeks are up and you have to get another shot there is a risk of getting pregnant; BUT I've also read that the shot stays in your system for a decent amount of times. Now my question is once you stop taking the shot and if you have yet to get your period at all and you don't know when the next time you will get it how do you know the chances of getting pregnant and is there a chance of getting pregnant even if you haven't gotten your period because of the shot?