the basic idea is UV is only good for a couple of weeks if it's effective killing the free floating algae. But all that algae gets put on the pond bottom, unles your filtration is fine enough to catch it. Most here don't use UV as we have bogs and lots of plants; these two things are more effective and provide more benefit for the pond.
All that said, most lights have a half life anyway and once you reach it, the intensity lessens such that you get a lot less benefit. I'd go by any hours rating and work the math that way. I generally don't like overpaying just because it's a name brand but it is true; you do get what you pay for.