Hey Lisa!I regularly check my lights with a uvb meter,they always exceed the 6 month recommendation and they very often last up to 9 months,before i owned a meter i was replacing my uvb every 6 months.If your light is failing to produce uvb,you should notice your chameleon spending a lot more time under the fixture as it tries to absorb as much of the diminished uvb radiation as possible its time to change your light.English is my second laguage and i try very hard to correct my self,i hope my grammar is o.k.