It all depend on you, I got a personnal trainer and for my height and weight, I got to do 30 minutes of non stop cardio.. Got to start with a heart rate of 145bpm for the first 4 minutes then for 1 minutes i got to get it up to 175bpm.. then so on till I reach 30 minutes..
I'ts amazing how hard it is.. [But its worth it.. It works!
But for your question.. Nothing is better then cardio so It doesnt matter how long you do it, at least you are doing some..