There's not much canon evidence for this, but we can infer and use comparisons to our own prison system to give us clues.
I'm thinking school cafeteria food, only a little worse. Maybe plain bread, meat, and water. Very basic and simple. These people wouldn't be given very good treatment after all.
What kind of food did they eat?
Probably three, perhaps one or two if the subject had been behaving badly (or if the guards/warden was particularly cruel).
How many times a day did they eat?
I could imagine maybe one *tiny* window completely covered in bars, for the cells along the outer wall. Cells along the inner wall would obviously not have windows.
Were there windows in the cells?
Perhaps a small cot with a flat, old mattress and a shabby blanket and pillow. I would assume some form of waste disposal system would have to be in place also, as the guards wouldn't want to risk letting prisoners out all the time to use the bathroom.
I highly doubt it. Most of Azkaban's inhabitants are considered very dangers and would most likely be kept out of the vacinity of other people whenever possible. However, if your story requires that prisoners have cell mates, I could see instances wherey you could make it fit.
How about proximity- would people share a cell?
Good luck with your story!