Like warpus, I'm a programmer and limit my work to 40 hours a week unless there's something really critical going on, which is about once per year. Most other developers where I work also do that, so it works out. If I were to work more than that per week, I'd have to be a lot more into what I was doing. Which might be possible with some jobs, but since it isn't at my current one, I'm not going to invest more time for no benefit (and it isn't the sort of company where I'd expect a noticeable promotion/raise as a result).
Technically, I don't have flex time, but de facto I do somewhat. My direct boss is very much of the "if you're getting your work done, I don't care where you're doing it" with time being somewhat flexible, but the higher management is very traditional. So what's worked out is that she's received a go-ahead for us to have some flex time, but it's not technically by the books.
The idea has been raised where I work of having bonuses for doing things outside the standard. Things like warpus mentioned with redoing low-quality old systems. That hasn't gained any traction yet, and most people don't want to stay till midnight and still have to work the next day, so the low-quality old systems stay unless upgrading them is put on a roadmap.
In terms of "divisibility", my job is somewhat divisible. The main reason I came in to work late at night last year, for example, was updating systems. After the first time (when the developers were needed in case something went wrong), this really should've been a system administrator's job. But our two system administrators are already swamped, so it's always the developers who do the production upgrades (this is supposed to change this quarter). And the one other developer on my team could do substantially everything I do, although our bus factor is likely 0 since with one fewer developer, it wouldn't be possible to meet our current goals. If I were to leave, our roadmaps would suffer, but eventually things would be back on track once replacement(s) were hired and trained. Which, knowing the management, would take quite awhile. Although much like when one of our DBAs left, there might be a need to reach out with questions for a month or two.
I rarely check my e-mail outside of work hours, in no small part so as not to set the expectation of being always available.
As for being able to work 1/4 fewer hours and share with a colleague? Provided we actually hired someone, that would probably work pretty well. 8 hours/day and 5 days/week is already past the point at which I can be optimally productive all the time, so I suspect the amount I got done wouldn't decrease much, and might well actually increase (it's the same reason I take breaks occasionally when I notice my focus disappearing). I've often wondered why American society is so enamored of the 40-hour (or more) work week, as productivity increases with technology. It would seem that, having achieved a good standard of living several decades ago, the average work week should slowly decrease over time while maintaining the same standard of living, much as it did in the late 1800s and early 1900s. Unless the goal is for everyone to work the same amount but have their own yacht, which I'm okay with forgoing.
or work a few more hours for a few weeks.
