Here's a great article on Salon that indirectly relates to VFX. It talks about the 40-hour work week, how that came to be, and what has happened over the last 30 years.
Personally speaking, it's been a relatively rare thing for me to work a 40 hour week. I usually put in 30 hours a week at a studio job (if I'm lucky) and then my teaching schedule is added on top of that. By this time of the year I'm pretty much burned out. My school holidays are coming up in six weeks, the same time when my studio contract runs out. Can't wait for some time off.
Generally speaking, I think most people in VFX are in a constant state of being tired and worn out. Every once and a while you hit a great project that's really well run, and you definitely saver those. I've been really lucky the past year or so, but even being on those great projects, the schedule can still take a toll on you.