How would you make time pass in an empty room?

[Duck Sandwich]

Work out. Which for me would include knuckle pushups. And shadowboxing. Except instead of punching air, I'd most likely be bored/insane enough to want to punch the walls.

Knuckle pushups + punching walls = scarred, bleeding knuckles. They'd bleed to the point where I'd write random profanities on the walls/floor by punching them.

Eventually, either I'd die from getting my wounds infected from contact with the prison floor, I'd die from the stench of 9 years worth of dried sweat/blood, or my knuckles would be so tough, that I could Falcon Punch a boulder without feeling pain. Whether or not I'd be able to punch said boulder into lava remains a mystery.

 

[Redlin5_v1legacy]

Fall asleep and dream. If not that, use my imagination and perhaps generate new story ideas whilst I rot in the room. Hopefully I can escape but I would be fine with nothing but my brain for at least four hours.

 

[joshuaayt]

I'd pretend, for a time, that I was trapped in that gigantic Cube, with traps all around me. I'd sit, and do math (To find the size of the whole thing, to escape!)
When I got bored, I'd ask the guards for some chalk, and draw increasingly intricate monsters on the walls.