It varies wildly from brand to brand and from model to model. That is kind of like asking how long a car lasts, some break down early, others last for decades. As with cars, it also depends on what kind of maintenance they get.
As Jason noted it will vary. It also will vary depending on if it is UV ozone vs. corona discharge. The modules of a corona discharge system should outlast the bulb of a UV based system by several years.
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