Does kombucha expire?
Pro-tip: How to age kombucha
How long does homemade kombucha keep? And how long can you keep it in the fridge once it's chilled?
It's safe to drink pretty much indefinitely as long as it's properly brewed and stored, but that doesn't mean you'll necessarily want to drink it after it's been stored for more than a year. That's because even if it's refrigerated, it'll likely taste like vinegar at that point.
What do cold temperatures do to yeast and bacteria?
Homemade kombucha is a live, real, natural product. It doesn't have yeast inhibitors and hasn't been pasteurized or watered-down in the same way that many mass-produced, store-bought, shelf-stable kombucha is.
Your home brew will have live yeast and bacteria, which goes into a state of dormancy when chilled. Once kombucha is refrigerated, it'll still continue to ferment, but at a very slow pace. So it'll still continue to get more carbonated and more acidic in the refrigerator as the yeasts continue to eat the sugars in your brew. It won't happen as rapidly as when you ferment at room temperature, but it'll be fermenting nonetheless.
Pro-tip: Age kombucha for complex flavors
You can actually use this slow, cold fermentation to your advantage if you like dry kombucha. Depending on the fruit flavorings used, kombucha can develop wonderfully complex flavors akin to dry cider, champagne or sour/wild beers. I often purposefully age bottles of my apple or passionfruit kombucha for 4-5 months to develop a delicious complexity. Age some raspberry or cherry kombucha to create a flavor similar to a traditional Belgian lambic beers/krieks!
Eventually, though, the kombucha will get to the point of acidity where it tastes like vinegar. In that case, you can use it in marinades or vinaigrettes.
Storing long-term? Burp regularly!
I'm typically not an advocate of burping bottles during second fermentation, but when you're aging your kombucha or keeping it in the fridge long-term, you'll want to release excess pressure in the bottles, so they don't get over-carbonated and make a mess when you open them. Burp them every 1-2 months until you drink them.