When to Replace UV Bulb in AC System?
UV lights in both HVAC and water purification systems should be changed yearly. 

This is also in the Honewell spec sheet for the UV2400:


  Lamp should be changed 1/year or every 9000 hrs to maintain its
  effectiveness




Many systems have a timer and will start beeping and/or flashing a light after 1 year to remind you to replace them.

It's not that they will just stop working after that time, but that their output is reduced over time and they're built so they have an effective output for a year. 

Should also be noted that the runtime is cumulative: if you only use this for 6 months of the year (and turn it off the other 6), you can go up to two years. The Honeywell documentation states 9000 hours (1 year = 8760 hours), which is typical of all water treatment UV systems (of which I have installed/serviced hundreds).