The issue isn't how long it's been, it's whether you (or your guests, family members, pets, etc.) caused the damage through other than normal wear and tear (e.g. tore the carpet; pet urine stains or red wine stains; etc.). If you did damage the exceeds wear and tear, then the landlord may charge you for its replacement or repair. On the other hand, if it is simply normal wear and tear, then the landlord may not charge you unless you have a lease which states that you are specifically responsible for carpet replacement (since it is legal for two parties, like a landlord and tenant, to contract so as to shift who is responsible for what).
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