It is not new that Google manual actions expire, it should also not be new to you that an expiration of a manual action does not mean the manual action will never return. It likely will, if you have not taken correction action.
Google suggests that you should never wait for a manual action penalty to expire, that you should take corrective action and submit a reconsideration request. But more importantly, I've seen this first hand, when manual actions expire, you notice it and then when they come back shortly after, you notice it as well.
A Google Webmaster Help thread has official written word from Google's Eric Kuan that manual actions expire and may indeed return to haunt you. Eric Kuan wrote:
It looks like the manual action that was applied by the webspam team on your site expired. Right now there are no manual actions affecting your site in Google's search results. However, even when a manual action expires, if the reason for the original manual action is still relevant, it's possible that the manual action will return later on.
So don't let manual actions that expire think you are safe, you may not be.
Forum discussion at Google Webmaster Help.
This post was written earlier this week and scheduled to be posted today.
Image credit to BigStockPhoto for hourglass