The time to call it quit: NEVER.
If you bought a given share,and it has dropped to near zero levels,please,save the dem thing,as your retirement or for your kids. It will definately come up one day! I hope it wasn't Enron share!
But you shall remember the LORD your God,for it is He who is giving you power to make wealth,that He may confirm His covenant which He swore to your fathers,as it is this day. Deu 8:18