While a set time limit or one minute per year are popular answers, and popular in practice, child psychologists recommend that the time out last as long as the behavior continues, plus a short time (like a minute) more. You could also include a requirement that the child apologize, identify what the problem was, explain what he or she will do differently next time, or some combination of those prior to release.To be most effective, the child controls the length of the time out. When the yelling/hitting/temper tantrum/whatever is over and the child is calm, then the time out can end. This teaches a child self-discipline and helps him/her develop an internal locus of control. Having a set time is (a) a punishment, not discipline, (b) provides the child no incentive to calm down, and (c) makes the child feel as if he/she has no control.