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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)

The sub 5 minute speed run of Super Mario Bros skips most of the game via warp pipes.

A more proper comparison would be the warpless category, which takes a bit less than 20 minutes.

Btw, for Mario 3 the 100% record is about one hour.

Imagine how disappointing it'd be to accidentally speedrun a game on your first play.

How many hours do you think it takes for a player to get to the point they can finish the game in 5 minutes?

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.

I think there is some youtuber that, recently?, played through every official NES game...

Edit: http://themexicanrunner.com/

Something which is fun to do in your free time becomes a chore if you have to do it reliably, on a deadline.