My understanding is the gaseous SO2 is the actual sanitizing agent, not the liquid. As long as it reeks and the liquid is clear and mostly free of chunks, I keep re-using it. When a gallon gets down to about half, or it doesn't look good, I replace it. In general this is about every 6 months, although I've used it up sooner. A few times I've kept a jug for 12 months, and it reeked as badly the day I dumped it as the day I made it.
For safety, never sniff ANY chemical mixture. Waft your hand over the mouth of the jug towards your nose and sniff very lightly. If the K-meta reeks, the gaseous SO2 is certainly present.
NEVER sniff deeply of K-meta, as it will burn your lungs!!! I run a fan when using it, and open the windows if the season and weather allows.
I should probably switch to Star San, but habit keeps me using K-meta.