1.) When your health starts to suffer, you are partying too much
2.) When you forego all your other responsibilities, you are partying too much. Skipping school, skipping work, neglecting your children, or your loved ones are the worst tell-tale signs of too much partying
3.) When you put shame upon yourself, your family, the company you represent, and your group of friends in a way that you would definitely not if you were sober, then you are partying too much.
Partying is a celebration of life; it is supposed to inspire you... not to destroy all other aspects of it.