Also depends on your method of encoding. I have Ripped Tank police From DVD 2 episodes and compressed them down to 49MB. Bearing in mind that when the VOB files were ripped and the WAV was saved it was equal to about 2,5GB!!!!
Also I can compress Full metal panic episodes (Great mecha anime, recomend to all) from 290MB down to 17-19MB! The quality on these files is not lost at all!!!! Sound + picture perfect
In other words m8, there is no sure MB to minutes ratio as its entirely dependant on:
The original File size (Of the file you want to convert)
The type of film (Cartoons, anime, manga, can lose many more frames without quality loss)
the quality you would like to acheive
Software used to rip and encode
I only have 1 64mb card but I can normally fit about an hour and half worth of anime on it!

)
Hope that clears it up a bit m8
C3DPO (XBOX LIVE TAG)