The truth is, when it comes to benefiting from aerobic exercise, frequency is totally up to you. Because cardio exercise is low intensity, you can actually do this 7 days a week if you really want to burn off all of your fat. If you really want to lose fat as quickly as possible, do as many days of cardio in the week as you can. Remember it is the only type of exercise that burns fat calories directly.

Frequency also depends on if you are in the fat burning stage or in the maintanence stage where you want to keep your body shape.

As a minimum you should do 3 days a week for 30 minutes when you are in the fat losing stage.

When you get to the stage you just want to maintain your body composition, you can do 3 days a week for 20 minutes a session along with your weights workout, and you'll be fine.

Ok, I know some of you want some more detailed information on exercise programs:

As far as particular exercise programs go for controlled, long lasting weight loss, I recommend two programs.

Tom Venuto's "Burn The Fat" ebook is the first recommendation because apart from containing a great section on exercise (he's a natural bodybuilder and personal trainer), he also has the best overall book on weight loss - from the many I've reviewed.

If you're only interested in the exercise section, then I believe Craig Ballantyne's "Turbulence Training" is best for you. His nutrition package may not be as comprehensive as Tom's book, but his exercise section is just killer. It is the best one I've seen that's targeted for weight loss. He even covers bodyweight exercises in his course (which is using your own bodyweight to simulate weight training without the weights).