The employer must determine the frequency of electrical retesting based on a number of factors including work practice and procedures, industry standards, frequency of use, work conditions, care, maintenance, and condition of the equipment, etc.  However, as a manufacturer we recommend at a minimum, hotsticks should be inspected and electrically retested at least every two years, rubber insulating gloves at least every 6 months, rubber insulating blankets at least every 12 months, and temporary protective grounds at least every 12 months. All this equipment should be thoroughly inspected before each use. If there are any signs, indications, or reason to suspect the equipment is damaged, it should be removed immediately from service for repair and retesting or disposal.

Links:

http://blog.hubbellpowersystems.com/5-steps-toward-keeping-cover-up-like-new