Kimchi I made in March still smells fine in July — when does old kimchi stop being good and start being a problem?

Two clocks run at different speeds and people conflate them. The safety clock barely moves once it is properly acidic, cold and submerged, because the acidity that made it kimchi is the same thing keeping other organisms out. The quality clock moves fast: crunch is mostly gone by month three in my fridge, the paste separates, and the flavour goes from bright to flatly sour by around month six. So is it still good and is it still safe have different answers, and the one you are actually asking is about quality.

Sour kimchi is not old kimchi, it is a different ingredient, and Korean home cooking treats it that way. Mine routinely goes a year in the fridge, and by then it is only for stew, fried rice and pancakes, where the softness stops mattering. What you have described — submerged, sour, fizzy, soft, no film — is exactly what a year-old jar looks like at month four. Keep it.

Pushing back on the kimchi-never-goes-bad line that always turns up in these threads. I had a jar go genuinely off — not sour, but slimy with a yeasty smell — after I left too much headspace with a lid that had stopped sealing. The cabbage at the top had been drifting above the brine for weeks. It was obvious when it happened and I did not have to agonise about binning it. So the rule is not never, it is not while it is submerged and cold.

Submerged, cold, smells like kimchi: eat it. Fuzzy growth of any colour, or a smell that makes you hesitate rather than wince: bin it. You will not need a rule to tell the difference when it happens.

One practical thing: the fizz means it is still making gas, so open it over the sink and go slowly. I have redecorated a kitchen wall with month-five kimchi brine. Also stop storing it in the fridge door — the temperature swings every time you open it, and that is where my jars have always aged worst.