What's the basis for cardio, i always do 40-60 minutes because i want to be more vascular - i keep reading i'm doing too much, after 20 minutes i should be getting off and getting on with my workout.

But i also read, doing large groups like chest and legs PRE cardio is better for fat loss and trying to do 40 minutes. I personally think, as long as i do it - it doesnt matter!

What's the scientific, bodybuilder reasoning for no more than 20 mins, from what i've read it's not too catabolic, you wont waste away!