The book suggests taking brood comb and holding it up to the sun. If you can see through it, it's fine. If you can't, it's time to replace. That's excessive to me.
I started switching out brood combs about three years ago, and I've noticed an improvement in the heath of my bees. I have less colonies that just appear to be dinks, for no other apparent reason. I'm not saying the wax is the cause of the improved heath, it could be from a wide variety of reasons: better beekeeping practices, better varroa management, better genetics, who knows.
I replace comb every 5-7 years, or throw out a few frames if they are those dark black kind. But I do it not only because of the potential of chemical buildups in the comb, but because I'm a firm believer that having colonies draw out wax every year is good for their heath. Just my take on it.