Unfortunately, without knowing what kind of head surgery
you have had or are going to have, it is impossible to say whether or not you can begin exercising again. It is true, however, that many surgeons use the two week mark as a time to resume normal activities after most simple surgeries. Your specific situation and variables will need to be addressed however, which is why the correct answer will have to come from your surgeon
Simple surgeries such as a tonsillectomy often require a brief rest period to allow full healing prior to a return to exercise or other strenuous activity. This is because all of these activities which cause exertion lead to increased blood pressure and can also cause strain on the incision itself. The goal of avoiding stress is to make sure that your body has some time to heal fully, and to make sure that the risk of re-bleeding after surgery is minimized. If you have had some kinds of surgery, this time of waiting will need to be increased however, and so please speak to your doctor
fully before engaging in any strenuous physical activity after any surgery of any kind.