When can a mare go back to work after giving birth?

Generally, it can take a few weeks - it does depend on the mare and, to an extent, on the foal. If the mare has to be separated from the foal to work, then you may have to wait longer than if you can, say, leave baby in a paddock right next to the arena or have him accompany you and the mare. (In fact, for training purposes, it can be highly beneficial to the foal to make him watch mom work - foals who observe their mother being ridden or driven remember it as a safe thing and are less stressed when it comes to their turn). A fit mare may be physically ready to work again as soon as two weeks after birth. Farmers would traditionally just let the foal tag along while the mare worked and many ranchers breeding foals as trail horses take a similar approach.

Obviously, if there were birth complications, you need to wait longer.