Remember it's 'High Intensity Interval Training' so I would keep it below 10 minutes, certainly to start with. If you're not totally knackered you're doing it wrong. Unless you're Michael Johnson any longer than that is normal cardio, albeit still interval cardio. Different principles then apply. You can do whatever you want but people do HIIT for the metabolic increase afterwards and less muscle burned, keep going for too long you're undoing the benefits of HIIT.
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