ive been told that the first 15-20 min is when your body is using its glycogen stores as energy (depending what/how much you ate before) then it dips into fat reserves for the following 15-20 min (crucial time for fat burning - again, depending on previous factors) after that muscle becomes the primary energy source. for fat burning, id say more than 20 min for sure (except on an empty stomach in morn, when 20 min is sufficient) but keep it less 45 min, just to be on the safe side.