Plumbing system winterization - when should I be conerned?

It takes quite some time to actually freeze and break pipes. You have to remove about 90 BTU to cool a gallon of water from 50F to 32F and freeze it. PVC has a heat transfer coefficient of about 0.03348333 BTU/hr/sqft/degree F. With some assumptions that works out to about 107 hours or 4 and half days of 32 temps. Colder temperature less time. Just to be safe I sometimes run the heat set on 50 when its going to be freezing for a couple of days. This way heat goes down to my holding tanks as well.