An hour and a half to two hours may be needed for a furnace tune-up. The biggest factor, though, is the furnace's age. If your furnace is brand new and kept clean, it might go more quickly.  However, no matter what the age if it is discovered that you require more substantial repairs, it will probably take longer. For instance, it shouldn't take long to relight the pilot light, but, it might take up to an hour or longer to replace or fix the electric ignition system and larger system parts like the blower motor.