Make sure cooked lentils are drained and cooled completely. Then, place them on a sheet pan, and freeze for 15-20 minutes to help them dry out and freeze slightly. Then, transfer them to a storage bag or airtight container, and freeze for up to 6 months.  When ready to use, you can either place the lentils in the refrigerator to defrost overnight. Or, if in a rush, thaw them at room temperature for 15-20 minutes or until mostly defrosted.