I like o_e_l_e_o's proposal a bit better, but with more time allowed (e.g. 3 activity period at least) to earn that merit, because if you're on a holiday or have something to do in real life for a while, you should not lose your merits because of inactivity.
Or, if we count only those activity periods, when the user has posted at least 1 post. This would not burn the acquired merits if someone is inactive.
As I said, I read this on another thread, and I've since gone to the bother of looking it up. It was stompix's suggestion here:
https://bitcointalk.org/index.php?topic=5092399.msg49017143#msg49017143. His original suggestion includes what suchmoon is suggesting: 1 merit for each activity period in which you display a signature. If you don't want to be charged the 1 merit, then remove your signature.