I used to pull 80 hours weeks when I was younger, but keep in mind I loved my job because it was a hobby before I got hired.
Several years and a few burn outs later, I've learned to pace myself. Rarely, but it does happen, I end up having to pull 12 to 20 hours; generally because something seriously went wrong or a deadline is in 24 hours. My ability to code optimally starts dwindling after ~6 hours, at 12 hours I'm probably no better than someone paid half my salary, and at 20 hours I might as well be a junior level programmer with the types of mistakes I'll start making.
Ideally however, you don't hack away at a problem until you have a solution; you work on other things you can finish while your mind works on the problem subconsciously. Too many times I've spent 10 hours trying to solve a problem, only to wake up one morning and go "wait... OMG why didn't I think of that sooner". C'est la vie.
Once I start getting bored with a project though, probably 30 to 40% of my billable hours are due to the frequent distractions I'll let affect me. 5 minutes here, 2 minutes there, 15 minutes there, etc; and it adds up pretty quickly.
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