Why would I hire Alice to work for 4 days a week when Bob is willing to work for 5 at the same price (assuming both are equally productive, even if AI has made that productivity extremely high)? So long as there is an oversupply of qualified workers, I doubt working hours will change. To do so would require social contract changes like for the standard 40-hour work week.
My expectation is that the social contract will change if most labor can be performed autonomously.
The question for me is whether only software engineering or white-collar work is affected, forcing us into different labor.
If instead most labor is affected, I am confident we will figure out a new social contract. Contrary to what some pessimists seem to believe, I very much doubt the evil elites are working towards a largely unemployed and poor population.
I could imagine that the norm will be two days of work per week, with five days off.