I went of depo after 12 months on it & it took 4 months after the last one ended for me to fall pg with DD2.
I got it when she was 2 weeks old & was on it til she was 18 months old. It took around 11 months to fall pg with DS.
So I do think the longer you are on it, the longer it takes to fall pg, but my cycle returned to normal within 2 months & with DS I wasn't actually TTC for the first few months, just not worrying at all. After about 10 months I was starting to worry, so we DTD every night for a month...It worked!
So I can't really say it was the depo that held it off, or just our timing with DTD.
Either way, as with everything it is different for everyone. I know alot of people who use it. I think it is trial & error, but I do believe you have to give it 2 injections to see how it really goes as the first one your body needs to get used to it.
As I said before, I love it & I'm going to stick with it.