I've always been puzzled about this, too because my goal right now is fat loss. I have his e-book and according to chapter 16 this is what he recommends:
1) 30-60 minutes of continuous activity (for gaining muscle, it's only 20-30 minutes)
2) 5-7 days a week of cardio (3-4 days of 20-30 minutes to maintain)
3) Moderate to high intensity (60% - 80% of your maximum heartrate)
4) Fasted cardio - working out on an empty stomach, preferably first thing in the a.m.
5) And variation is key - don't do the same cardio activities over and over.
He also suggests only doing as much (or as little) as necessary to achieve results. This decreases the adaptation response from our bodies when it gets so used to doing the same thing over and over again. This way, when you're body "plateaus" you have lots of different ways to change things up: length of time (30 minutes to 40 minutes,) type of activity (running one day, kickboxing another, etc), time of day (morning vs. evening, etc). You get the point.
If you have a good base of muscle to begin with, he says that you're more likely to lose muscle from a poor diet than from overdoing cardio.
I hope that helps - if you're debating about purchasing the book, I refer to it as my bible for nutrition and exercise. You won't regret it!
Terri