High-intensity interval training (HIIT) entails doing short bursts of exercise with short periods of rest back to back for up to 30 minutes. I would recommend 15-30 minutes for a workout but really going for it!
Low intensity steady-state (LISS) cardio on the other hand entails long periods of low-intensity exercise such as walking, jogging, cycling, swimming, etc. - there is no specific time for this form of exercise but 30+ minutes is a ballpark figure.
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LISS, on the other hand, being much in lower intensity can be performed much more frequently. This is why you can "walk" your way to fat loss results, by upping your step count or adding a morning walk or jog into your routine.
Both can be considered as tools in your armory, with LISS practiced more frequently (daily in the case of walking) and HIIT practiced 1-2 times per week around resistance training, or on a 3 day on 1 day off rotation as I discussed in idea 1.