I've referenced several manufacture sites (such as Firestone & Slam Specialties) to find any verbiage on their site's but I was unable to find if air bags for a hot rod should be changed after so many miles? My understanding is airbags are somewhat similar to tires, they can dry rot, bulge or blow over time. But, like tires, there is a determined life expectancy, but I am unable to find what is the typical life of an airbag and at what mileage are they typically changed?
Treat them like other suspension components (and with a similar lifespan). The reason it's so hard to get an answer on lifespan is that it's very dependent on the installation and the other components. If it's installed incorrectly - which happens a lot - then the bag can rub against other parts of the car, and won't last as long. Use cheap hoses, or a dodgy compressor, and again the system won't be great.
So, in summary, replace them when they break (or when the rubber looks in bad condition). No need to throw away money until that point.