there is no easy answer to that.
even if we know that its standard definition content... there are so many variables such as framerate, quality, compression and color depth that can make a huge impact on file size.
if you are figuring standard dvd quality.... about 1 hour per 2 gb
if its extremely compressed and poor quality video then as ss2 suggested it may be possible to get a few hours per gb. thats a big IF though.
look at your files.... pick one out and see how many mb/gb it is and how long it is. divide the two to get you a mb/min value and then you will have your answer.