I've worked days and days and days...
BUT
That includes travelling (often me driving).
It does not involves hours in front of a desk. It involves lots of different things, from driving, to unloading, to fixing PA's to mixing, very physically demanding things.
But at least each one of them is a break from the other, no matter how hard.
None of them has involved so much constant focus, which is why I'm a learner at what you guys do.
Having said that - I learn really fast.