I should actually be working 8h a day, but most of it is spend not working. If I'm honest I'm probably working more like 3h a day even though I enjoy my job.
It fluctuates based on workload, but I find myself working anywhere from 4-5 hours a day to basically nonstop during my workday (9 hrs). I do think most people are really only capable of doing "good" work, meaning being at their most productive, for about 3 hours a day though. The rest of the time is spent slogging through and putting out mediocre work, just to get it done.
The same for me (but 8 hour workday). Honestly, I couldn't do the job if the working non-stop days were the default. I am wasted after such a high-stress day, so I need it to fluctuate. I also don't feel bad on days where I do less, because I know I do a 110% on the other days. A workday is simply too long to be productive the whole time and the workload usually varies.