How often should you use exfoliating body wash on back acne?

Daily. The washes containing salicylic acid may be irritating if overused. A wash containing benzoyl peroxide may be better tolerated, plus you get an antibacterial effect. One does need to be careful, however, because the benzoyl peroxide can bleach colored fabrics.
Daily. If not irritating a salicylic acid prep could be used daily.