How many GB does an average movie take up on an mp3?

None.

MP3 is an audio format and hence not really suited for movies.

If you're talking about MP4, however, it depends, since the bit rate is not a fixed part of the format, but variable. But, typically, it will be about 700MB for 'acceptable' quality on a 90 minute movie, though this depends on your personal threshold for video artifacts.