The Merit Source allocated quota personal maximum can never be surpassed by acumulating unspent sourced Merit. To give a simple example, If a Merit source has an allocated quota of say 300 monthly sMerits, and does not send any within the month, he’ll still have 300 sMerit in the bag a month later (and not 600). They don’t exactly decay, just not stock up above the allocated threshold.
On a finer grain detail, the sMerits are refilled not on a fixed calendar day, but rather exactly 30 days (down to the second) after any given sMerit TX is made.