The question about choosing a work that has been chosen as the general book club's selection is well founded.
I think that one year is an adequate quarantine. I observe that out of the 28 different posters on this thread, 7 joined MR before July 2009, that is 2 years ago (25%).
If you want to be surer of not repeating the choice in front of the same group of members, you can choose a 2 years period. That would leave out only one member.