I can regularly pull a 15 or 16 hour session (yeh I know thats not ideal!) and pretty much 'zone out' anything that involves interacting with the OS during that from start to end; at a conscious level it just floats right in one eye and out the other
On 10.10, even when I'm not getting hit with something different that does make its way thru and ring an alarm bell, there's probably umpteen things being given a subconscious double take, so even if I'm not aware of it, my eyes are probably being worked all that much harder as my brain seeks its endless visual confirmations. About an hour or two is all I can manage.
Maybe that's why I really don't mind 'the look of it' in screen shots anywhere near as much as in actual use; I'm looking at a picture, my brain isn't kicking into 'using OSX' mode, my eyes aren't doing their usual constant scanning of every pixel on the screen etc.