This is the quiet trap in reptile lighting: a UVB bulb can look perfect and be functionally finished. The phosphors and gases that generate ultraviolet degrade faster than the parts making visible light, so output slides downward month by month while the tube still lights the enclosure cheerfully. An animal can sit under a glowing bulb and receive too little UVB to make vitamin D3 properly, with the consequences — poor calcium use, weak bones over time — arriving slowly enough that nobody connects them to a bulb that 'still works'.
Working Replacement Intervals
Follow the maker of your specific bulb first, because output curves differ, but the common working guides are these. Fluorescent UVB tubes are typically replaced around every six to twelve months. Compact screw-in UVB bulbs fade fastest and are usually changed sooner, which surprises owners who assumed the newer format lasted longer. Mercury vapour bulbs, which combine heat and UVB, generally hold useful output longer but still have a finite life measured in months of daily use, not years. In every case the clock that matters is hours lit: a bulb on a long summer photoperiod ages faster than the calendar suggests. If you cannot remember when a bulb went in, treat it as due — an early replacement costs little against the alternative.
Measuring Beats Guessing
The only way to know what a bulb is actually producing is a UV meter, which reads the ultraviolet at the animal's basking distance. Keepers with several enclosures often find one meter pays for itself in bulbs not replaced early and animals not left under dead ones. Measure a new bulb at the basking spot, write the reading down, and re-measure periodically; when output falls well below the new-bulb figure, the bulb is done regardless of age. Distance matters as much as age — output falls steeply with every extra inch, so a bulb mounted too far away may never have delivered enough even when new. Screens between bulb and animal cut output further, which the meter will show you plainly at the basking surface.
Habits That Keep the Schedule Honest
Write the installation date on the bulb's end cap or fitting with a permanent marker the day it goes in — the simplest system, and the one that survives memory. Keep one spare bulb in the house so a replacement is a swap, not a shopping delay during which the animal sits under nothing. When you change a bulb, note what the animal's behaviour does over the following days; increased basking and appetite after a change is a common and slightly sobering sign of how faded the old one had become. Storing each bulb's install date and type with the pet's other records in the KinsLe Pet Passport puts lighting on the same footing as vet visits — scheduled, dated and impossible to leave to guesswork.