20-45 minutes of cardio a day is good for you and helps you gain muscle, is more cardio better? What I have found with my own personal experience, is that there are four issues that crop up and hinder muscular gains when you do too much cardio.
Short term fatigue : If you play an intense game of soccer or basketball and workout right afterward, you will have a lousy workout and your muscular development will suffer. Your body can only take so much and if you are exhausted from running all over the field, there is no way you can put enough intensity into your workout to make gains. However, if you wait 3-5 hours to recuperate before lifting weights, you weight workout will be just as aggressive and successful as if you hadn’t done the cardio
Total exhaustion: cardio till you drop. Depending on your conditioning and intensity level, after 3-16 hours you will just konk out. If you have ever done endurance sports, you know exactly what total exhaustion feels like. For me, doing an olympic length triathlon which is about 3hrs of intense cardio puts me on the edge – 4 hours would definitely put me in the total exhaustion category. For my long distance cycling trips, because the intensity is much lower I can go 12-14 hours before reaching total exhaustion. Once you reach total exhaustion, any weight workout you attempt will not be successful. Not only will you be too tired to aggressively push the weights but after the workout, your body will be too busy repairing itself from the brutal cardio session to be able to build up your muscles
Repetitive motion injuries: Often its not the cardio itself that’s the problem, it’s the injuries. Do anything for more than 4 hours and you are asking for repetitive motion injuries, doesn't matter if its running, rowing, swimming or cycling. Once your joints are in pain, any possibility of muscle mass gaining weight workouts go out the window.
Overtraining: This is the biggie. If you get too much cardio, the major muscle which has provided your locomotion is essentially doing a mega-rep, low weight workout. If you do 6 hours of squats, you will suffer from severe overtraining and lose both strength and muscle mass. Likewise, if you do two marathons a day you will lose leg mass which leads us to this amazing study