Very little if any.  A certified lightning protection system should last as long as the building itself. 
The system is designed to withstand numerous lightning strikes without degradation.  It should also withstand any weather conditions the roof
itself could withstand.  We recommend that the system be visually inspected every 5 years, to assure rods and cables are fastened tightly to the roof and walls. 
If you have had any repairs or modifications done to the exterior that may have
affected the system such as re-roofing, the lightning protection system should be re-inspected
and re-certified by a licensed agent.  Since you have paid for the system, it's a good idea to ensure that it is still effective.
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