I have also wondered if it is really necessary to add more Kmeta if the carboy is not being opened. I can think of two possible reasons we might need to add more:
- If there is a small leak, and oxygen is getting in.
- If the Kmeta (free SO2) is gradually being "used up" (bound with O2) already in the wine. In that case, when the Kmeta is used up, there might be a need for additional Kmeta
It seems that if the Kmeta binds all the oxygen in the wine, and no more leaks in, we wouldn't need any additional Kmeta. But since I can't be 100% sure that neither reason above applies, I am leaning toward adding Kmeta every 6 months as a precaution.