Is there a way to know how many spendable merits are going to expire soon ? If not, it should be included in profile if possible.
For now there's no way to know when they'll decay and that's why you are advice not to hoard it but rather send it out to well deserved post and comment. It can be decayed anytime when the forum administrators wish thou I don't think it will happen soon. Nevertheless, it has no meaning keeping it for long in your possession.