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.
But the signature is still visible on all the posts they made previously. They could remove it manually before going on vacation if they don't want to be charged.