I went onto depo in the summer of 1999, and was on it until my July 2007 jab wore off. I went onto the pill after a very short break of no contraception and was on that for about a year, during which time I had barely-there periods (needed 1-2 tampons a month).
I went back on it in Jan/Feb 2009 and then had my last jab in early Dec 2011. When this wore off in Feb 2012 I went back onto the pill as I wanted depo out of my system but wasn't planning on TTC until I was married in Oct. When I went onto the pill this time I had 'normal' (for me) periods right from the start.
I came off the pill in May 2012 and conceived DD within 5 months (my cycle lengths were all over the shop, even if the flow was normal, so I was on my 6th cycle when I fell pregnant).
I was definitely ovulating for most of that time (bad OV pain plus positive OPKs).
Apparently the average time from last jab to conception is 10 months, which I was almost exactly. However, I know I'm really lucky as it really was quite straight forwards for me (I was also almost-36, so not exactly young, fertility-wise).
You'll hear some post-depo horror stories. Some may be a coincidence (i.e that person would've struggled anyway, but the obvious thing to blame is depo. Impossible to know if that's the case though).
I'm proof that it doesn't always take forever / cause issues.