When you feel ready is the simple answer. However, there are a number of issues involved, and the time will be different for each couple. If the delivery was particularly traumatic or you have had stitches resuming a sexual relationship may take some time. A new baby also makes so many demands on new parents, so some couples feel they don't have the energy or time for a physical relationship.
Before you resume a sexual relationship remember to consider contraception. Breast feeding in itself does not provide absolute protection and you do not have to start menstruating again before you can conceive.