Do you want to know the exact amount of days that you should do cardio?
Runners are ones that could tell you what they have done during their training, how many days, what they ate, how much sleep they got each day and so forth- and it works great for them. I'll list 2 major key elements that make all the difference in the world when it comes to cardiorespiratory performance- the safe, non-over-training way.
1. Development is about specificity. What are you doing cardio for? Are you going cross-country, around the block or losing weight? Be specific to your demands. If you are losing weight, go with what you can handle- whether 15, 20 or 30 minutes at the moment is better than nothing at all and work your way up gradually to a comfort zone.
2. You want to be great each time out and you don't want to over-train. Check your resting pulse each morning. If you are ever 9 beats higher than your normal resting heart rate- you need to relax that day and you continue to relax until it goes back down.
I hope this helps-
Train smart and hard : Not just hard