Mary, there is no exact answer because there are too many variables such as the qulaity of the image, the amount of movement, sigle or double-sided disc, etc.
You can surely squeeze a lot of video onto a disc,but the result may be equivalent to an audio CD with a splotchy, blocky formless accompanyment.
Gary Kleiner
Learn Vegas and DVD Architect
http://www.VegasTrainingAndTools.com