I was given the depo shot sometime mid cycle, when will it become effective?

Depo: If you get the shot within the first seven days of a period then it works immediately. If you were not at the time of your menses then you need to wait a week before unprotected sex.

I would say about 30: Days.Since we don't know wether you ovulated prior to or after the shot.

It's probably: effective within 24 hours but it can only be guaranteed to be effective if the first shot is taken within 5 days of the start of the menstrual period. To be safe, you should use a back up method until your next period. (I am assuming you had a pregnancy test before you were given the injection).