My UV lamps last 2 years. I haven't seen green water since I
installed it about, hm, 5 years ago (on my third lamp now, its
now in its 2nd year).

Aqua UV 40 Watt. Big ol' ballast. 2" pipe. 1 gl/sec pump.
3000 gl. It had the little, inline ballast but that died in
the second year.

The rubber bushing thing doesn't last long, at least not my
first one (UV rays tore it up). The next lamp came with a
purple-colored seal and that's still fine.

If you're using the UV only to combat green water, I don't see
any reason to replace it unless the water is turning green-ish,
and the lamp is visibly "on" (i.e., the ballast is working,
and plugged in).

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SLOTHEAD