It differs from person to person. It took me about eight months before I had periods again. For some women it's more, for others less. Unfortunately, you'll have to be patient and wait for your body to adjust to being off the shot.
Keep in mind that your last shot is good for three months so your body is only just now beginning to readjust. The DP shot is one of the birth control methods that is recommended for women who don't intend to get pregnant for this very reason- because it can sometimes take so long for our bodies to resume their normal menstrual periods.