How far out should my garage heater vent through the wall?

I often see large accumulations of frost under the garage soffit where the heater is vented. It is important that the warm moist air that is coming out of the heater vent is not entering the attic space through the soffit. If the vent is too short, the warm air rises up into the attic where it condenses when it hits the cold air there. This condensation can lead to rot and eventual weakening of technology rafter tails and roof sheathing. Leaks due to ice damming are also possible. It is best to extend the vent out beyond the eave a few inches.