How long can you grind?

My absolute longest grind-session is I think, 18 hours. :x
That was only once though, and a while back.

Pretty sure I passed out after that though.
xD

A solid 18 hours? Jesus, my one time going 14 hours was hell on my eyes, and I can barely go 9 now. How in the hell did your eyes not fall out of your head at 18 straight hours?