I replace my filter on a quarterly basis, which is the perfect interval for me. After a month, the filter still looks like brand new, so changing it monthly would be wasteful. The 1500 filter seems to do an excellent job of maintaining a good indoor air quality. The only "problem" I've had is very slight variations in the actual size of the filter...sometimes the new filter fits in perfectly with no resistance, and other times I have to force it in.