I do mine every two years typically though if I have one on hand at my annual service, I might do it then. I find them spotless generally so for me, two years works.
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If you don’t drive a lot of dust they last quite a while.
You’ll know when it’s blocking as your airflow in cabin reduces.
All good - I got the local LR dealer to do it. Changes the engine filter, cleaned the pollen filter, cleaned the engine, aircon and power steering radiators and turned a really beat up key into a brand new looking one - for $250.