This is a good response to that question, in case anyone wanted the q&a paired together. 15 would be the social average — some people might work more, some less, but that would be up to them. This is also presupposing a change in the way the economy effectively functions, in the sense that 15 hours would be the social average for necessary labor, and after that point you’d probably see tons of people participating in “unnecessary” labor out of interest. There’s an idea in socialist theory that basically says once you reach a point far enough along after capitalism you’d just start seeing a blurring of the lines between what’s defined as “work” (or as “a job”) and what’s defined as “a collective activity”, especially with regard to the arts, science, etc.
At the very least, a transition towards (eco)socialism will require a vast shortening of the workweek and a reduction in overall advertising/consumption, whether that mean a 15-hour week or a 20-hour week. The main point is that people are overworked, we can meet everyone’s needs feasibly with less hours anyway, and the over-emphasis on extraction and accumulation (which is in part fueled by a tediously long workweek) is having disastrous effects on the planet. We can both accomodate the needs of the planet AND expand the political horizons for the great majority of the population, but it will require the fundamental defeat of capitalism and the establishment of a new ecological workers’ democracy. It’s a big project, but so worth it in the long run.