'03 LE. 4-cy.
Going by the "book," (the maintenance book toyota supplies with the new car), I first needed to replace the cabin air filter at 10,000mi in the camry (and several other models, also). Then at 15,000mi (but only in the Camry), then at 20,000mi.
I didn't look further. Asked the local dealership service "adviser" about it (seems screwy to me), but he just shrugged and told me to follow the book.
Wouldn't mind so much, but the filter costs $27.95 at Toyota, $25.95 at CarQuest and Napa "doesn't stock it yet."