In my own thought there is nothing like resetting after a month is over, it is obtainable that after a 30 day period from the time a source sends a particular merit from his source balance, the used merits are refilled. I've read somewhere that it is an automatic process to refill for used ones after 30 days and the unused one remains there until used before the countdown for its refill starts. It is only that their max source sendables for a month is capped at their allocated amounts. A source can correct me if I'm wrong.
Unused merits for merit sources will decay. That means if you have 100 source merits and you spent only 50 merits on the monthly cycle, that means the remaining 50 merits will decay. Next month you will get only your allocation of 100 merits. I don't know when exactly refill decayed merits will be added, but your remaining merits won't be added to your source merits for the next month.
Thanks for this clarification and it justifiably should be so inorder for the merit sources not to hoard their merit allocations, so it is better for them to distribute everything within their timeframe. It wouldn't be fair for merit sources to hoard merit especially when they didn't earn it from other members because I believe that there are enough posts that should meet their quality requirements.
However, for general users, it will not decay for now. But the forum has the full right to decay or remove your merits anytime they want, and it's clearly stated on the merit page. So you shouldn't be confused about merits decay.
I don't see any good reasons why we should worry about the time when our merits would decay when we can give them out just like the merit sources, wanting to know about the decaying duration would imply that the holder perhaps planned to keep for a long duration. This forum's merit are not useful outside the forum so it is better to share as soon as possible unless you don't see posts that meets your quality standard which will be surprising.