I have always heard no more than five years but it probably all depends on the condition. I'm sure this is one of those topics where you will get a different opinion from everyone you ask so I am interested in following the email string to see the response.
I took an advanced beekeeping course by a respected local commercial beekeeper last summer. He's big on chemical free beekeeping, and recommends changing out every 5 years as well, and here is why; He sent some of his comb to Penn State several years ago to have it tested. Figured it would come back completely clean. Instead it came back with fairly high levels of the chemical found in dog flea & tick collars! He asked a dairy farmer nearby if he used anything like that, and found out he sprayed his cows with it. Apparently, it washed off the cows, was absorbed by the plants from the soil, collected by the bees, and worked into the comb. You never know where they forage,and what they bring back, so changing it out makes sense to me.
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.
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