From Pregnancy.org's "Ask the Midwife" expert answer to a similar question:

"Some women can get pregnant as soon as they miss a shot by less than a week, even if they've been on Depo for years. Others take over a year to start again even if they've only had a few shots."

To help determine if you are ovulating I would suggest using a Basal Body Temperature chart, like the one found here , to help you detect when and if you are ovulating.

Good Luck!