If your initial calibration was done in your home flying area (and you're sure there was nothing around you that would influence the local magnetic field), then you don't have to recalibrate unless you get a warning to do so. If you've chosen a new spot to fly from close to your home location, my advice is to use the Magnetometer built into you iPhone/iPad and a free app to check around and see. As Richard mentioned above, if you're traveling cross country and want to make sure you're referencing Magnetic North for that area, then do a recalibrate.