It looks like fasting intervals can sum up to about 12 days per month. I don't recommend going all the way to 15 days. Even half of that should bring some benefit. Nice thing about intervals, a person should be able to start low and work up to it. For example, if you managed to fast twice for three days each month, this would be comparable to 20% calorie restriction. Like a workout, you could piece intervals together. I had to break my fast early this weekend, so I'm fasting again next weekend. It is intermittent fasting for fitness buffs. The intermittent fasting folks say that you can eat as much as you want in between fasts and still get the same benefit. Athletic types can fast without worrying that they are running too short of calories. For extended fasts, two week recovery condition applies at this time. Be safe. Nice thing about fasting recovery interval is that you can still workout. In fact, working out is recommended throughout.
Some may look at these calorie numbers and say that the program is to strenuous, but short-term fasting does not produce physical damage, so that a heavier calorie load is possible during fasting than during physical activity.
What is an extended fast? It's like an extended workout. Fasting 3 days or more burns several thousand calories, which is comparable to running a marathon or biking a centuray ride. I was thrilled by the improvement in my running time after my last extended fast. Fasting is the athlete's secret.