Assuming it was paid training, 8 hours a day, 5 days a week
160 hours gives about 90 minutes per game. For the NES that's probably more than acceptable, many games were very short (Super Mario Bros can be finished by speedrunners in just 5 minutes for an extreme example)
They likely did it as a group, so together to some extent. When someone got stuck I suppose the others would give hints and tips. Still a difficult task, but slightly easier than going at it alone.
160 hours gives about 90 minutes per game. For the NES that's probably more than acceptable, many games were very short (Super Mario Bros can be finished by speedrunners in just 5 minutes for an extreme example)