It is cold and dark here in hell. Rough, frigid stone chafes my naked skin, and I am shivering. I think the Men Upstairs are due to pump heat into the room soon, but I’m not sure. It’s impossible to keep any real sense of time in this complete blackness. All I really know is that I am here (wherever “here” is), and that I am alone in a cold, dark room..
I have paced my cell--three steps wide, four steps long. I suppose at least that is fairly generous; I can stretch out when I lie down to sleep. I never sleep long, though. I always wake up shortly after I doze off, shivering on the abrasive floor.
I hear--nothing. Not right now. When they turn on the heat, I will hear the soft chuffchuffchuff as a compressor pumps warm air down toward me. The heat unit is above me -- I think far above. I sense this more than I know it, but I know it all the same. If I wanted to hear something now I could scrape my fingernails on the stone beneath me, but i’m all done with that. My nails are down to the cuticles from it, and I’m sure I’ve left trails of blood in the floor from previous hours of scratch scratching away. I have no desire to leave more blood for them to find when they are through with me.
I think you could make the sentences a little tighter. For example, "sense of time" instead of "real sense of time", "blackness" instead of "complete blackness", "skin" instead of "naked skin".
Cuticle is at the very base of the fingernail - if he's (she? I am assuming the narrator is male for some reason) scratched his nails down to the cuticle, he has no nails at all left. Is that really waht you meant? I'm not sure that's as plausible as, say, scratching his nails down to the quick.
I was also a little confused by the temperature - he talks about the heat coming on, but he also talks about waking up shivering. That could be a little clearer.
But interesting.
I didn't have a problem with the voice being a bit wordy, but I did have a problem believing that this story would ever go anywhere interesting. Your first line, "It is cold and dark here in hell", implies that he's not going to get out of there within the scope of the story. Being locked somewhere dark and unpleasant for an entire story isn't my idea of reading pleasure. Just my impression though.
On a more particular note, I think that present tense narrative is okay as long as the story takes place within a "moment" of time. A moment is a length of time short enough that the character doesn't actually have any unusual or important experiences during that time, so it can be pretty long.
Outside of that moment of time, I've never seen it used successfully in any story. I have seen it used in otherwise successful stories, but it was always a Jonah.