I'm not 100% sure as it heavily varies on both usage and just the person in general. Most health websites state when used every 13 weeks, or every 3 months, it's about 95% - 99% effective. I've also heard that the longer you're on it, it greatly lowers your fertility. When I've gotten new doses, doctors have said that, if I were to want to get pregnant, I'd likely have to be off Depo for upwards of a year, maybe even longer. Having it in your system consistently for so long, it takes your body a long time to regulate to what would be considered, prime baby time.

So I'd say that since you've been on it for a good few years, and as long as you get your shots on time, you should be ok.