All depends on the water in your area, the maintenance of the water heater by draining the heater once a year to reduce the buildup of sediment and controlling the possibility of high water pressure. Thermal expansion tanks are a good plumbing device for controlling excess pressure which affects not just the water heater, but the whole plumbing system.
Simple logic would say —- the average life expectancy for a gas water heater operating on mains pressure is about 12 years. It seems after that kinda time the anode that protects the cylinder from corrosion would probably have disolved away and allowed corrosion to commence on the cylinder walls which will eventually end up with a leak. The glass or vitreous enamel on the clinder interior deterorates with heat, age and aggressive water and results in a leaking cylinder. Hope this answers your questions.
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