For me (as a programmer) it really varies a ton. I used to put in insane stretches, due to the medication I needed to take in the past and that is how I got used to things in college.
Now I work more regularly, but still can put in a solid 6+ hr day most of the time, and yet some days… yeesh I’m lucky if I can get a third of that. So I work more on other days to compensate.
Probably around 4h over a 8h day on average in an IT engineering job.
My brain tend to procrastinate and wander around a lot, random questions come around and I look them up, maybe 60% related to my job, but not directly to my current task. Overall, unless I have a boss, as good at my job as I am (only happened once), watching over my shoulder and asking what I did the past hours, it’s not a problem.
I think this wandering allows me to be as efficient during my active hours as some people are during 8h. And the 4h searching random stuff makes me more knowledge, including in my job field.