Ear and microphone pads of headphones should generally be replaced every 6 to 24 months, depending on the intensity and frequency of use as well as hygiene requirements.
Conclusion: There is no general rule, but the ear pads should be replaced at the latest when they start to lose their appearance or functionality. Anyone who uses their headphones intensively on a daily basis should ideally plan to replace them every year or so; infrequent users can extend the intervals as long as hygiene and comfort are good.