To me, it includes breaks to refuel and use the restroom, and if it's more than maybe 6 hours, will factor in the time to scarf down a fast-food meal. Also, any time this discussion involves "X hours" it's undoubtedly being rounded and estimated, and it will involve a hundred different little variables like traffic, road construction, the driver's tolerance for speeding a bit, etc. Also, don't forget that it's common, but not formal, so there's no single way people are taught, and different contexts will require different levels of accuracy.