Depending on the bitrate(new formats can allow lower bit rates) and format you use.
DVD uses MPEG2. I average about 3 gigabytes per hour in this format(my card has a pre defined dvd ready bit rate it records at. I am using it with media center.), but this will depend on the quality setting you use(more quality = higher size). You have a practical limit for DVD(over that limit, the dvd player may not be able to keep up).