When would a person need to be put on anti depressants and how severe would their case have to be?

Depression: Whenever deperession bothers you enouhg to complain, you should be started on antidepressants.

If pretty severe: Antipressants can be helpful when formal symptoms of depression persist for 2 weeks or more. These include feelings of hopelessness, loss of appetite (weight loss), disturbed sleep and others. But the decision isn't mechanical. Most importantly, antidepressants should be used in conjunction with "talk" therapy which is often and ideally sufficient. Other means help greatly, too.