Not sure on VHO's but most people say roughly once per year on MH's. This does fluctuate, some say 8 months, some say 18 months. What most agree on is that youneed to change them before you start to visually see a change in the quality or colour of lighting.
We change ours at the 12 month period.
Hey, how's the anemone doing?
Best,
Rob
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