If you notice the brine level falling within the first 7-10 days of fermentation, I recommend making up some additional brine (dissolving around ½ Tbsp of pickling salt per 1 cup of water). However, after that point, the cabbage usually has enough bacteria to be considered a "safe environment" for fermentation without the risk of mold/slime. Though I recommend topping up the jar with a 2% brine for optimal shelf life.