If you aren't smelling any weed smells, you have no reason to swap it out.

To answer this, you need to know a little about how the filter works. Filters are full of activated carbon, which binds to organic molecules and prevents them leaving the surface of the filter and hitting the receptors in your nose. The places where this may occur on a molecular level are called BINDING SITES. There are only a limited number of these contained in the filter, and the number depends on the MASS of carbon and the PARTICLE SIZE of the bits of activated charcoal in the filter. When all these binding sites have an organic molecule stuck to them, the filter is useless.

Lifespan depends on number of plants, how aromatic the strains are that you run, and what your temps are; all these factors compared to the MASS OF ACTIVATED CARBON contained in the filter determine the life.

A smaller filter (=less exposed active sites/ surface area of carbon) won't last as long under the same conditions as a larger one that can physically bind a larger mass of aromatic molecules.

If you have someone you can trust who does not live there, have them be your 'nose' since you may be unable to smell weed smells in your home because you are used to them.

Let your nose be your guide! Hope that helps some.