What about this?:

http://tinyurl.com/2bn7a5

That type of filter should be listed for a certain number of gallons, rather
than a minimum time. It is a charcoal filter and when it is full, it is
full. The use for an aquarium for a year would be different than for a
10000 gallon pond. I have seen koi killed using a carbon filter that ran
out of steam. If you are going to use one of these, it would be a good idea
to have a chlorine test kit for a swimming pool to check the charcoal filter
to be sure that it is still working. RTB

The other KHA in our club has done amazing stuff with carbon filtration. He
purchased 2 whole house carbon filters with a spigot in between, he runs
them in parallel. He tests from the spigot periodically, but not a pool
tester, not really sensitive enough we found. If it registers, he pulls the
filter from the 2nd and puts it in the 1st and a new one goes in the 2nd.
That way he's always covered. This works really well for a large pond.

For the rest of us, the item chatnoir listed, is probably do able, but
again, one would have to test it, probably each time until they figured out
just how many gallons it can do. People have made these from supplies at
hardware stores cheaper, I believe. Seems one could run a couple of these
cylinder units similar to what my co-KHA did with the whole house units. By
using a quick disconnect to test the 1st cylinder periodically.

Wow, after typing that, it seems like a lot of work... I think I'll stick
with dechlor as long as my water supply sticks to chlorine. ;-) ~ jan
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Zone 7a, SE Washington State
Ponds: www.jjspond.us