It's really simple. Take 600 and divide it by the number of minutes of your project. The result is the total bitrate (video + audio) you're aiming for. In your case, 600/120=5Mb/sec.

Raw PCM audio eats about 1.5Mb/sec, while AC-3 is only about .2Mb/sec. So depending on your audio compression or lack thereof, your video data rate could be either 3.5Mb/sec (probably mediocre quality) or 4.8Mb/sec (good quality if encoded carefully using VBR).

I can't take credit for the elegantly simple computation above. Someone else on this board has mentioned this a number of times.