The length of time a propane tank lasts on a griddle will depend on the size of the tank and how often you use the griddle. On average, a 20-pound propane tank will last between 18 and 20 hours of continuous use on a griddle. However, this can vary depending on the heat output of your griddle and other factors. It's always a good idea to have a spare propane tank on hand, so you don't run out in the middle of cooking.