From Rabid Jackalope:
Video Disk Space Calculator is a free, simple application designed to calculate necessary disk space for video editors. As of version 1.1 it also can calculate length available for a specified disk size.
Simply select your desired video codec, enter the length of the footage or available disk space, and VDSC will display the space needed or length available. A printable chart of many codecs and their respective data rates is also built in.