I delete all members who seem to drop off the face of Earth without warning, staff get slightly longer to come back before deletion. Also I do not consider anything before joining forums; I join then consider is my nature. I'd usually sign up for the sake of it and bookmark it after a few days if the community is good.
I think 6 months is a bit on the short side for deletion, but I want to keep the member:post ratio clean, and my whole forum knows that.
But the question is: what do you do when your most active helpful member suddenly flies to Mars overnight, never to be seen again?