friend key currently are given to full subs 1 or more per month subbed with a max hold of 5 keys. Giving them out is easy, so is getting them for players who are subbed year round.
friend keys are in a sense unless, 1 week of a gifted sub, then snapped back into crystal, when a account uses a friend key they can never use a friend key on that account again(this is the kicker).
I suggest that instead of a one and done make it were a account can use a friend key every 2-4 months, 6 months at max(wall-clock).

Why would we want to do that?

-1. Exploit-enabling feature.

I assume friend keys exist to give new players a taste of subbed life in hopes that they would subscribe. Giving repeated free access would seem to circumvent that, and does not support the development of the game.