Since they're essentially a symbiotic culture of bacteria and yeast (SCOBY), they won't "stop working" as long as the grains are regularly replenished with milk, or kept in the fridge to slow down the fermentation process.
Since they're essentially a symbiotic culture of bacteria and yeast (SCOBY), they won't "stop working" as long as the grains are regularly replenished with milk, or kept in the fridge to slow down the fermentation process.