I think it's a bit more of a complicated answer than just X minutes. A lot of it depends on what your goals are, why your are exercising in general, and why you are doing each exercise in particular. And you get into a situation very quickly where you'll find that the right number is going to be very different for everyone.
For myself: Well, I like to eat and I like to eat a wide variety of foods, some of which are more calorie dense than others. To balance the way I eat, I do 30 min of hard cardio on the machines 3 times a week. But that isn't my only goal. I also like racquet sports- so I play racquetball for at least an hour a couple of times a week for fun. I want to get leaner and faster on the court, so I interval train jogging 3 times a week for about 45 min. I live near a beautiful redwood-covered mountain range that looks out onto the ocean, so I go hiking sometimes. And sometimes my SO and I take a walk along the ocean just to appreciate the view (with rent what it is here, that view costs us a lot of money!). All of these are cardio exercises and for a wide variety of reasons they are all add up to the "right" number of minutes for me.
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