UNH Extension puts good T8 fluorescent tubes at 20,000 to 30,000 hours, and that is the number to plan around. Scope matters before you trust any figure on grow light lifespan. The tested hours cover fluorescent tubes, not LEDs. So the answer comes in two halves, and one half rests on much better evidence than the other.
How you switch a fixture matters more than the brand on the box. Picture a shop light flicked on and off five or six times a day, every time you pass the shelf and want a peek at the seedlings. It wears out sooner than the same fixture running one long stretch. Every start is hard on a tube. Long runs stretch fluorescent tube life and scattered short ones cut it.
The gap between those two habits is wide. Cheap tubes switched on and off all day can drop to about 10,000 hours, which is half or a third of what a good tube gives you under steadier treatment. Rated hours assume the lamp gets to run, not that it gets clicked awake a dozen times before lunch. Buy the better tube, keep it in one block, and you get two or three times the service out of the same fixture.
LEDs are where the confident numbers dry up. No university or peer reviewed source publishes a consumer LED degradation curve. None gives a replacement interval either. The big hour counts on the packaging are sales figures, not tested ones. Heat is the part that has been measured. Efficacy falls about 10% as the junction temperature climbs from 77°F (25°C) to 185°F (85°C). That is why the fixture carries a heat sink, and why you should never box it in against a shelf.
None of that makes an LED panel a bad buy. It means the grow light lifespan question has no tested answer on the LED side yet, so treat any promise of a decade of service as advertising. Keeping the panel cool is the one lever you hold. A fixture with open sides and a little moving air around it stays nearer the efficacy it was rated at.
Run every fixture on a timer in one unbroken daily block. Steady switching protects the tube, and the timer kills the habit of flicking the light on for a two minute peek.
Every setup should run on a timer. Pick your daily block, set it once, then leave the switch alone for the rest of the season. Give the fixture some air too. Leave the top and sides open instead of tucking it tight under a shelf lip. A small fan pushing air across the space keeps heat from sitting against the housing. Moving air is cheap protection for a fixture you paid real money for.
Plan grow light replacement by hours on the clock instead of by the calendar. At 14 hours a day, 20,000 hours works out to about four years, and 30,000 runs close to six. Write the install date on the end of each tube with a marker. No cited source gives a dimming threshold you can test at home, so those logged hours are the best guide you have. The timer does most of the work.
Read the full article: Grow Lights for Indoor Plants: What Really Works
