i've worked with a lot of people in a lot of places, and I think you are kidding yourself if you think most people get 8 hours of work done every day. I'm not saying nobody can do it, but most can't and don't. And that's not a problem. Most people are going to need breaks throughout the day. I suspect many of us here check out hacker news at some point during the day. In moderation that is fine as well (IMO).
For me personally, sometimes I put in 10-12 hours a day for a month or two and then I back off to 4-5 hours for awhile. I can do it in spurts, but 8 hours everyday for years at a time sucks. more than 8 is just unreasonable. If we're being honest I suspect most people do between 4-6 hours of work a day. Perhaps if they weren't forced to be in the office for 8 hours, they could fit that 4-6 hours of work in 4-6 hours instead of 8.
The cleverer managers I've worked with typically include a factor of ~ 70-80% on worker office time. So in a typical 8 hour work day, 6 of those hours would be productively spent on a task at hand.
For an engineer who is sitting at his or her desk, focussed on a single task, I think this is reasonably accurate. Obviously there are a lot of things in an office environment which can further conspire to reduce the amount of productivity much, much lower.
For me personally, sometimes I put in 10-12 hours a day for a month or two and then I back off to 4-5 hours for awhile. I can do it in spurts, but 8 hours everyday for years at a time sucks. more than 8 is just unreasonable. If we're being honest I suspect most people do between 4-6 hours of work a day. Perhaps if they weren't forced to be in the office for 8 hours, they could fit that 4-6 hours of work in 4-6 hours instead of 8.