It depends on the video format (mpeg, avi, mov, etc...)
It depends on the dimensions of the video (320x200, 1280x1024, or even larger)
and it also depends on what is being filmed (see the descriptions of how video encoding works)
5 minutes of a cartoon in a very small format and 5 minutes of an action movie in a large - HD format do not have the same size at all (like, really not)...