To each his own, but if I see that I have an hour slot, I prefer to do something like a code review or emails. Even the most menial programming task can easily have a surprise that extends it beyond an hour, and I feel something like anxiety knowing I have a hard interruption coming.

Anecdotally, I do like to have discrete, 4 hour blocks of time, and this is something that seems pretty common (though not universal) among the experienced devs I’ve worked with.