It is based on the severity of your climate and the outdoor temperature. In a more severe climate with 6,000 HDD, it should run only 34 minutes. A more severe climate requires a larger furnace to handle the coldest days. The best types of energy audits use infrared thermography and blower door infiltration tests plus a detailed inspection by a technician. Infrared thermography uses a camera with heat sensitive film or special scanners.
Hot areas appear red and cold areas appear blue.
This is an excellent method for detecting voids in wall insulation. Pictures and scans from both inside and outside are most effective. A blower door test is used to determine the amount of air leakage into a house and where the air leakage is occurring. A large fan is positioned in a window or a door and it sucks air out of the house. This negative pressure draws outdoor air in any leaky areas so they are easier to detect.