We would expect a regassed air conditioning system to last more than 12 months before requiring the procedure again, which may indicate a leak. However, a leak check should be part of the regassing process, so you may wish to seek a second opinion. It is a sensible precaution to use your air conditioning throughout the year, if not necessarily keep it on all the time, as leaving it switched off for several months of the year can cause components to seize and become inoperable in the summer.