The generator will retain its polarity indefinitely, power on or not, unless it's handled roughly. Take it off the truck, drop it on the floor a few times, and you'll need to re-polarize. Otherwise, don't sweat it! Polarizing the generator creates "residual magnetism" in the armature that will produce about 2 volts DC at the armature terminal, when the generator spins up, even with no wires attached. This starts current flowing through the regulator, and back to the field coils to start the charging process. That's why a generator-equipped vehicle can be push-started, even without a battery installed. Don't try that with an alternator, it can't produce voltage without at least a little juice from the battery.
Jerry
