Essentially - with UV lighting , you get what you pay for.
Cheap ==> elcheapo Chinese knockoffs often sold as home brands / shop brands , avoid these they are a false economy as they don't perform as well , and don't last as long .
All UV sources exhibit UVA and UVB output decay over time with extended use on a daily basis (in my case all my lights are on a 16hr photoperiod. ( On 6am , off 10pm).
If you have a good quality brand UVB , use the manufacturers guidelines on when to replace.
Basically - If in doubt , play it safe and replace every 6 months. I use a permanent marker to mark the installed date on all my lights (UV, spotties, household) so I can check very easily how old they are.
Most will recommend replacing the
UVB light every 6 months , the better quality brands (ie Arcadia) might say once per year.
If you have access to a Solarmeter 6.2 microwatt UVB / cm^2 meter, you can check the UVB output and perhaps if it's STILL inside tolerance keep using a while longer than specified by the manufacturer.
I recently bought a model 6.2 meter
http://www.solarmeter.com/model62.html and discovered my old 26W UVB200s were now essentially equiv in UVB output to 26W UVB150s.
An excellent investment if you have several pet reptiles and are running a UVB for each.