There's not enough merit sources to be sure, but I think the awarding of merits tends to poop out near the end of each month. If I'm not mistaken, merit sources get new allocations of smerits at the beginning of each month. They probably use them up before the month is over. That's my guess, anyway.
No, it's a rolling replenishment, i.e. today you'd get back the sMerits you spent 30 days ago, and so on.