I tend to pull an hour and then do something else unless I am in the zone and I can go up to about 3 hours and then break and come back to it.
8 hour days normally go like this when working with at clients place.
hour 1. opening up stuff, replying to email, grabbing a coffee, having a chat with co-workers(makes for a better working atosphere and you might learn something).
hour2. working on project, coding some, asking questions about the spec(if there is a spec, if not then asking questions that the end client or the account manager hadn't tought to ask)
hour3. repeat 2 but throw in a coffee and a chat and maybe a quick surf while you drink your coffee.
hour4. repeat 2
hour5. lunch time get up and move around people
hour6. repeat 2
hour7. repeat 3
hour8. start tying up loseends last coffee of the day etc send those emails you have been meaning to do, have a look at your bug list and fix a couple.
now you have done about 3 to 4 hours coding and plenty of other work.
I think part of the issue here is how you define "work". I probably spend about 3 hours a day actively writing code, but I would still say that I'm working 12+ hours a day -- the fact is that most of my time is spent coming up with ideas for how to solve problems and convincing myself that the solutions I've found are in fact the right solutions.
Lawyers understand this -- they'll bill you for time they've spent thinking about your file while having a shower just the same as they bill you for time they spend at a computer writing up legal documents. Unless you're purely a code monkey who writes code according to directions and never thinks about what he's doing, there's nothing wrong with counting "think time" as time you spend working.
Edit: See also my comments from November about procrastinating: http://news.ycombinator.com/item?id=81366