There's probably no such thing as too long with a really great therapist, because all of us, young and old, have room to grow. But there is such a thing as long enough, which would vary a lot from one person to the next, and would depend on such things as regaining emotional equilibrium and confidence, and whether the cost, in terms of money and time, might be better used in other ways.
This is a highly individual call. Have you had a quiet conversation with your son about how he feels stopping or continuing the counseling? He's old enough to have some sense of his own well-being. If he wants to continue, you might ask him if he has a sense of what he's still hoping to gain. If he's just going because he likes having a wise counselor to talk to, that is lovely if you can afford it, but may not be necessary.
If you get the sense that your son has made your desired gains, it would be reasonable to tell the counselor that you need to stop, but perhaps will call her again in the future if the need becomes apparent. You can ask her, if you wish, whether she has specific reasons she would advise continuing therapy. She may tell you something important, which you would want to weigh in your final decision. But the decision should be yours, based on all reasonable factors.
Be sure she knows how much you have appreciated the help she's given your son – but it sounds like she knows that already.