Rest is an essential part of any muscle-building program. In order to grow, your body needs time to recover and rebuild from the tough workouts you put it through.
Rest is an especially important part of your weekly ritual if you're a beginner or intermediate lifter. Your body may not be accustomed to working so hard so often, so you need to give it time to recuperate. Otherwise, you run the risk of overtraining or causing yourself injury.
I want you to perform at least one cardio workout per microcycle in Project Mass. As you go through the program, however, you may notice that your body feels stronger and more capable of doing extra work. When that happens, you can choose to add additional cardio to each microcycle.
If you've spent a few years in the gym and know how your body reacts to hard training, then you can be more aggressive in your cardio protocol.
No matter your level, though, be sure to listen to your body. Bodybuilding isn't a sprint, it's a marathon. There's no point to killing yourself for a few months just to lose all of your gains during a month of recovery.