Should the AVERAGE (Foundation, Intensive, Extensive) mesocycle phase last 2 or 3 weeks before unloading, assuming the athlete had a training age of 3 years.
I think it really depends on what kind of things you put in your mesocycle and how quickly you adapt to those things, but between 2 and 3 weeks is about right for most people. You can plan a 3 week mesocycle and always shorten it later if you need to.