It happened to me in 1958. On my way home to California from my summer job in Zion in my '50 Merc my headlights got dimmer and dimmer an hour out of Los Vegas somewhere around midnight with 500 miles still ahead of me.
I found an open truck service garage in Vegas when the engine died, and they advised me of my problem although they were too busy to help. So, using an Army surplus flashlight I removed the generator brush dust cover, got some toothpicks to put behind the generator brushes which had worn to the nub.
Got a push start and finished the trip around noon. My answer is also 1 hour.