Think of burn mp3's in this manner: 1MB=1 minute of music (depending on how high a bit rate the mp3 is, typically this is true of 128 bit rate though).
So if you have a 750 MB blank cd, that's 750 minutes of music, so roughly 12 hours of easy listening. Just don't try doing this where you burned the mp3's to a cd and then expect it to play in your car's stereo system. Most car systems won't recognize the cd and won't play anything as the file format is not in aiff form (typical way music is put onto cd's). If you do have one of the portable cd players that can play both, just get a car adapter kit and *poof* lots of music on one cd. Hope that helps.