There is really no way to tell how long a toothache will last. If it is simply the result of biting or brushing too hard, it may subside immediately or shortly after. If it is caused by something more serious, the pain may come and go, but never really go away completely.
Chances are if you are experiencing tooth pain or sensitivity, there is probably an underlying reason that needs attention. Putting off going to the dentist will most likely exacerbate the problem, leave you in pain longer, or even progress into something else even more painful. When in doubt, schedule a visit with your dentist so he or she can identify the cause of your pain and provide some appropriate treatment options.