Theoretically, if the battery is not allowed to discharge, it should last forever. This is more the case with the MT920 battery than it is with the CTL920, as the MT can cycle up to 500 times, the CTL only 100. And when Citizen advertises these Eco Drive watches as 'Never needs a battery', they're lying to you. In an oblique fashion perhaps, but lying none the less. Take the 'battery' that an Eco Drive watch never needs, out of the movement, and put it in a dark environment. I've replaced batteries in Calibre 2100 movements many, many times.