How Much Cardio Is Enough ?

I do 20 to 30 minutes of cardio 5 days a week after about 45 minutes of weight training.  I feel awesome afterwards and that's what I think a person should go by. Don't do so much cardio each day that you're NOT looking forward to doing it the next day. I think a person needs to do 20 minutes of high intensity cardio five days a week or 30 minutes of moderate intensity cardio five days a week. Unless you're not used to doing any kind of cardio yet. Then you should start with like 10 minutes and work your way up to that. 
I would suggest just doing cardio to where you're looking forward to doing it and you feel great afterwards. I personally would rather do 20 to 30 minutes a day, five days per week instead of two 1 hour cardio sessions per week. That is a psychological thing and many people think about that and it just makes them dread it.  Spread it around a bit, let yourself enjoy it! I actually enjoy doing 30 or 45 minutes of cardio but because I'm doing weight training before had I tend to keep it around the 20 minutes of high intensity mark .