For now they do not expire, but in the future they may, that's why you are adviced not to hoard them, but to give them out to other users, so that ranking up process never halts.
There is no point in hoarding sMerit; keeping it yourself does not benefit you, and we reserve the right to decay unused sMerit in the future
This is a quote to the message you see when you are about meriting a user, and this is exactly what theymos(admin)says about hoarding sendable merits and on if they can actually be decayed/expire.