An academic work is a creative work. And as such it can't be counted in hours. When you do research, you can't really stop working, your brain has no off switch. That's why it is very important that academics loves what they do.
What you can count is hours spent actively doing stuff related to your job: writing papers, reviewing papers, emailing, preparing courses, etc.
And yet the correct answer will be "depends on the week". Some weeks (okay, very rarely) it can be as low as 20 hours, because your mind in too preoccupied by other stuff like your personal life. The good thing is that this is okay, because some other weeks (most weeks in practice), it will be a lot more. Typically if one has a deadline for a paper submission, the two weeks before, you can bet that 60 hours is a minimum. I didn't count but this week-end only I probably worked for more than 10 hours.