Interesting study. I would be neat if another pair of twins could duplicate the experiments to broaden to data base. By the way, Scott Kelly and Mikhail Korniyenko fell 25 days shy of a full year in space. Musa Manarov and Vladimir Titov DID spend a full year in space. Valery Polyakov spent fourteen months aloft on a single spaceflight, the all time record for a single flight.
I would think that 2 or 3 days would be fun. Anything more than 7 and you are paying me the big bucks.