UV lights in hot tubs should be changed every 12-18 months. The bulb can burn out beyond that timeframe.
UV light aids in destroying water-borne pathogens including algae, bacteria, cysts and viruses such as E.Coli, Cryptosporidium, Giardia and many more however does not negate needing to continue to use a sanitizer in your pool or hot tub such as chlorine or bromine.