Hi, i just saw a TV show called Myth or Science. It is part of Nature of things by David Suzuki wich is a top canadian very respected scientist. But like many researches it was short and only on a few subjects.
First in a lab they proved with mices that exercising 3 times weekly had a positive effect on aging.
Than they proved it in humans studying skin cells because they have a real short time turn over.
But they know many only have little time to train so they did try with 3 days weekly of 1 minute full out exercise fallowed by 1 minute rest repeated 10 times in a row. They got positive results.
So 60 minutes weekly, 3 days of 20 minutes that consist of 10 one minute bursts was considered enough to produce positive results to improve.