If there is no points balance, yes I think it it will expire after 18 months or so of inactivity.
If there is a points balance, card will remain active whilst there is points in the account (points have a life of 3 years after earning), and then account becomes inactive sometime after points expire (i.e. no points balance)
In either instance, as KF is free to join, no big deal, just rejoin after previous account expires.