Cabbage is popular in the spring, so try turning it into sauerkraut! But how do you know when fermentation in complete?
Fermentation naturally stops as acids accumulate to the extent that further growth of desirable bacteria cannot take place. The bubbling ceases and the color should have changed from green/white to tan. It should be tart but still have a firm texture. The brine should not be cloudy.
If mold is present, has a slimy texture, or has a bad odor, do not eat. Fully fermented sauerkraut can be stored in the refrigerator, canned or frozen.
Source: https://extension.psu.edu/lets-preserve-fermentation-sauerkraut-and-pickles