Personally, I am changing out combs every two to three years. with more and more proof that chemical residues are present even in wax foundation, it seems wise to give them something clean. Besides, as you start looking it just seems that the bees thrive on clean, light comb. For me, the desiding factor was when I had a swarm move into some empty hive equipment that had several older frames ready to go. The bees moved in and droped new comb off the inner cover right next to the comb that was already present- they never touched my frames because they had an option.