I wish I still had the links handy, but there's over a century's worth of research out there showing that it's a bad idea to go over 40 hours in a workweek for any extended period of time.
Of course, everyone's different, but I've noticed I have 4-6 hours of productive time in me per day for knowledge work (e.g. programming). I'm guessing I could get multiples of 4 provided there were long breaks in between, enjoyed what I was doing, and wasn't constantly being interrupted.
Perhaps this is true in a corporate environment, but some of the more productive periods in, say, open source development, or research, are pretty much whole life affairs. Einstein's Annus Mirabilis, for example.
Of course, everyone's different, but I've noticed I have 4-6 hours of productive time in me per day for knowledge work (e.g. programming). I'm guessing I could get multiples of 4 provided there were long breaks in between, enjoyed what I was doing, and wasn't constantly being interrupted.