you spend a vast majority of the day in the "fat burning zone" (probably more than 23 hours), so no need to worry about 15 minutes here and 5 minutes there is too short. if you do short mini exercises it's better to push as hard as possible during these to increase the metabolic rate.
when it comes to duration and benefits it all depends on how fit you are, what you do, and what your goals are. quite a lot of research nowadays has showed that short high intensity intervals are beneficial for increasing VO2max. google for tabata and you'll find plenty of information about this.
--- non-racers. the emptiness of those lives shocks me ---