Product review (4 stars): Wicking filters
We buy these regularly for the humidifier.  We use the humidifier during one of our kids illnesses and then throw the filters out afterward.  I'd say with daily use they last about 2 weeks before getting pretty nasty, but it probably depends on the quality of your water - ours is suburban city water.  Its nice because the humidifier stays relatively clean - all the buildup is on the filter.  Having said that, the cost of feeping these filters around is probably more than I care to know.  Only real gripe is that the filters don't always seem to fit quite right at first, but as they get saturated, they compress into place under the water tank.