The thing is that the human naked eye is not as sensitive as plants are to the strength/power of light, so even if your lighting looks asĀ bright as it did 10-12 months ago the light that is beingĀ emmited will beĀ lower than when the bulb was new.
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This will make a difference to your plants, itĀ will not be that noticable in a low tech set up but will be more notcable in a high tech set up. WIth my LED tiles I can set them at 85% and then anything higher up to 100% is not noticable to my eyes. After swapping my T5 bulbs after 12 months use I would notice my plants pearling a lot heavier than they did with the 12 month old tubes.
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Like Mike said using bulbs till they blow is fine, your plants wont die off they will just adapt to the lower levels of light and grow slower.