If you are using mpeg2 (the video codec used in dvd-video) you should use no more than ~ 2 1/2 hrs of video per single layer dvd, which holds about 4.3gb of data. That works out to 1.72gb per hour of video.
This is an approximate measurement, nothing set in stone here. You can push that up to about 3hrs of mpeg2 per dvd disk, but the quality will definitely start to degrade as the bitrate falls. Mpeg2 is not a great codec for low bitrates, though some mpeg2 encoders do a slightly better job with that task than others.