How Much Time Will It Take For Depo To Wear Off And Allow Pregnancy?

My wife was injected with contraceptive , I think the name is Depo and since four years now we have been trying to have another child without success. We have had several tests to eliminate infections and low sperm count. We were told that my wife has fibriods but the position may not inhibit conception. The question now is how long does it take for Depo to wear off to allow pregnancy. What next do we need to do?