6 months would work just as well; perhaps 1 year is a bit too long.
As far as inactive members are concerned, it's already been agreed that their puzzles can be moved by Administrators after some time has elapsed; they can also post the answer (if they know it, then they'll get a point!) and move it to Solved. If no-one knows the answer and the poster is inactive then there's not much we can do, but I think there really are not many members who've posted puzzles that are still unsolved but who are now inactive.
So what do you think, Ultra, Otto, Allemano, PJ, Porridgehead / other Admins if there are any...?
Shall a rule be introduced to cater for it?