The system suspends subscriptions until it believes you have read the article (or thread??) that resulted in the last forum post notification.
Several of us have been suggesting that the "Subscriptions" page, ie. http://www.boardgamegeek.com/geekforum.php3?action=controls&... show you which threads you are subscribed to that are suspended (waiting for you to read the new content before it will send you further new forum post notifications) because it's easy to loose track.
But as to your question, as far as I know, subscriptions to at least regular forums and game forums do not time out (at least I've had long running subscriptons with months between posts, and I still receive the notification).