The effectiveness of a UV lamp decreases with age, so they can stop being effective long before they actually burn out. There should be a manufacturer rating o

The effectiveness of a UV lamp decreases with age, so they can stop being effective long before they actually burn out. There should be a manufacturer rating on the lamp, which will depend on its composition and other things. As an example the lamps I use have a rating of ~8000hrs, about a year, but to the outside observer they'll still look functional until they blow after 2-3 years.