I think 5 hours of 100% engaged, mentally intense work is about all that can be expected from a person in a day.
After 5-6 hours of mentally intense work, there are diminishing returns.
Therefore, I think about 6 hours/day are what should be expected from a developer, including 30-60 minutes of meetings/admin work. Though some companies might need another hour or two-- hopefully such extra time is not mentally intense and is more socially oriented work (such as meetings).
"Youtube: How Hard Should You Work? - Jordan Peterson"