Q: How long can I leave my truck sitting without it being plugged in before the diesel starts to gel up? Its currently 15 Degrees

To be blunt, there is no specific number that we can give you. You have to understand that diesel fuel gels in colder climates due to the amount, and types, of paraffin wax that is used in the manufacture of the fuel you are using. All refineries will use different formulas for their diesel and it will be formulated for your region, and its typical weather patterns. The cloud point of the fuel is the point to where the temperature has dropped low enough that the paraffin waxes cannot be dissolved any longer. The gel point will typically be 6 to 10 degrees below the cloud point. So, if the cloud point of your fuel is 18 degrees, when the fuel reaches an actual temperature of 8 to 12 degrees, it will begin to gel. Sorry for the lengthy answer, but there really is no short answer.