By default, wheel mice move up or down three lines for each notch you roll on the mouse wheel.
While you can adjust this setting within the software that came with your mouse, you'll typically be able to choose between the following settings:
3 Lines, 6 Lines, Screen, and None.

If you have one of the larger monitors and are using a high resolution setting, you'll probably find that you'll want to use a setting other than these. You can fine-tune how far your mouse scrolls with each one-notch rotation by editing the following Registry value:

\HKEY_CURRENT_USER\Control Panel\Desktop\WheelScrollLines

You can set the WheelScrollLines to a value from 0 to 4294967294.
A value of 0 disables scrolling altogether, and 4294967294 configures your mouse to scroll one page with each one-notch rotation.