How many hours a day do you look at a computer screen, and does it hurt your eyes when you have black text on a white background? What background color/font color combination is easiest on your eyes?

I am merely asking because I stair at compilers and excel documents all day and I am convinced my eyes are being fried from it. I was reading maddox the other day and he talks about how he keeps his background color black and font yellow and large for that very reason, he stairs at white pages on his comp all day and it is killing his eyes.