How many pixels are scrolled on a website with a mouse wheel down event?
Solution 1:
Many mouse drivers let you set the distance scrolled by the mouse wheel, so there is not a standard distance. I'd try your code out for a while and pick a distance that keeps you from scrolling all day but doesn't jump a mile at each scroll. You'll kind of need to "feel" it out. Get friends to give feedback, it helps to let a few hands touch this kind of thing.
Solution 2:
I have noted that in Google Chrome - that is 100px per mouse scroll