I've heard many things, generally the usual is to run for anything over 20 mins,

however, two of my roomates are in a kin program, and according to them, if you go all out 3 times, at 30secs, it is just as good as running for 15-20 mins,

granted going full out for 30 sec, 3x is an incredible amount, and will almost kill you, but it's quick,

just my thoughts