How long can you grind?

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?

I'm not sure myself. xD
It's certainly not something I'd do again, or even try to. I was pretty motivated at the time though..

& I think the chinkiness of my eyes helped keep them in.
f3.