What a difference a new bulb makes.

[Delphinus]

Verrrry true. Lights shift their spectrum.

However some lights useful lifespans last longer than others. It depends on the brand, the type, the ballast you use ... etc. Higher-K bulbs (>10000K) and actinics have a much shorter useful lifespan than lower-K bulbs. Some are useless after 6 months, others I have been able to sqeak out 12+ months. But even on those a side by side comparison of a new bulb and an older bulb, same brand, same wattage, same everything in all other respects ... should be enough to convince anyone that we need to stick to a fixed schedule of bulb replacement.