MGS, Minimum Guarantee System, runs on a monthly basis.
If you miss one month, you can still attempt to earn it the next month.
MGS is a good way to earn a bit of money, but as you said, it is time limited, and well, $200 is not enough for me to stress myself over it.
I am also of the impression that if your story earns more than $200 a month, you do not get the earnings + $200, but just the earnings.
The minimum guarantee system is to ensure that you get a minimum of $200 a month, but if you earn more than that on your own, then it just vanishes.
Even so, I think it is a nice incentive for new authors to earn a bit of revenue on their novels.
Be aware that the non-exclusive contract through EMP does not give you MGS.