Heat and chemicals are the worst enemies of the elastomer material. The quality of the elastomer material also plays an important part in the speed of deterioration. I would say that when you have to start wondering about if it's still good, that it's time to replace it. Because yours is going on 40 years old I would say it's time. You can look around the edges and if you see any cracking or missing pieces it's definitely time. A new harmonic balancer (or rebuilt) should be part of an engine rebuild. They don't cost that much and they're a PITA to replace when the engine is in the car.
Cars from the Southwest U.S. usually don't have much rust, and they don't have much left in the way of upholstery, dash pad, rubber bushings and grommets, fan belts, and harmonic balancer elastomer material.
Because the quality and life-span of modern elastomers is much better, and where you live, a new good quality harmonic balancer should last a long time.