Back in the good old days of the Stone Age we used to work 4-6 hours per day, based on the anthropological evidence we have (as Historia Civilis points out[1]). That seems to be the amount of work humans naturally slot into when left alone. Instead we work 8+ hours per day per 5 day work week. In European countries like the Netherlands, and the Nordics, for example, that's slightly below 7 hours per day (on average, assuming a 5 day work week), so they're getting close to the range we used to work. But a 4 day work week still seems like an almost utopian idea to achieve politically, despite all the insane productivity gains we've made over the last 100 years thanks to automation that makes even a four day work week seem laughable - we should probably be thinking of a three day work week at this point.

[1] https://www.youtube.com/watch?v=hvk_XylEmLo