My personal experience with friends/family accounts is that the miles do not expire, even if there is no account activity greater than 18 months. Periodically when you log into your OnePass account, you have to agree to updated terms/conditions that advise you have the 18 month mileage expiration, however, no reports yet of that being enforced.

You can always do small things to gain mileage with minimal cost, such as OnePass shopping, iDine, etc.