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.
If all source merits are not used in 30 days, all of them will be decayed and new source merits will be refilled, it's the easiest case for understanding about source merit decay and refill.
I am not a merit source, but through posts of some merit sources when they wanted to merit a post but no longer had merit, they wrote like they had to wait for some days before their source merit refill that I think only partial refill, not like 30-day source merit refill. So I guess the source merit refill will be do day by day, not like waiting exactly 30 days for a refill.
Like if you send 10 merit today, next 30 days that 10 merit will be refilled.