depends. a rebound to what? assuming you mean a woman; then there isn't really a time where your not a rebound. stand up for yourself and not let yourself be a second choice. no body should be put on the back burner, that's just another way of being used. and I'm sure your better than that.
It really depends on the person who had recently undergone a breakup, and how well they are coping with it. As a good rule of thumb, I would recommend three weeks as the absolute minimum, and then wait even longer depending on the seriousness of the relationship. For example, if a girl had a boyfriend for less than a month, two to three weeks may be plenty of time, but, if it lasted over six months, she may take months to really get over it.
When a person is hurting from a breakup and goes to be with someone new to help relieve that pain, then that's a rebound. For it to not be a rebound, then the person would have to no longer be hurting from the previous breakup and be ready to move on.