I think once upon a time it was like 3 months... then SE started the "Return Home to Vana'diel" thing some years back and said, "hay guyz we wer lieing lolz we still hav ur accounts even if u quit a yer ago surprize!!1" I think it's a year now, but I'm not completely sure.

I kinda have the feeling SE wants to keep old accounts around for quite a while so that they can give a very large grace period for people to come back in. Probably a good policy since growth has slowed down (to be expected in a 5 year old MMORPG).