Filters will last 12 months. The filter is a an inexpensive investment since you can't guarantee the quality of the CO2. 10 years ago I use to fill CO2 tanks, using dry ice converter tanks from 1941! A couple of homebrew guys could tell our gas was giving their beer an off taste.
You can find the filter at:
http://www.micromatic.com/product.php?pid=770SG-L1300
Mike Godwin, Micro Matic USA, Inc. Northridge, CA