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I've had it last months in the fridge* so long as I keep the air out. Toward the end, it'll start tasting more soy-sauce-y, but it never got anything even remotely bad-tasting.
* I bought a really big tub of it once because that's all they had and it was cheap.
Since I only use a tablespoon or two here and there, I generally have mine well more than 6 months with no change in taste or appearance. Not sure this is the "right" answer, but it seems to work. I would probably have them longer, but we tend to lose electrical power often where I live and so I've been on a sort of forced refrig purge with bad weather mid-winter and mid-summer.
I've had a tub of it in the fridge for months too and it pretty much looks the same as when I bought it so I think I'll go with it. Thank you both!
Agreed!! Since miso is a fermented and living product and very high in salt it is very difficult for any other microorganisms to take up residence in it and cause spoilage.
I'm pretty sure mine has been in the fridge for 2 years. It still smells, tastes, and looks the same, and I've read in multiple sources that it should keep at least a year. The one think I make sure to do is cover it with plastic wrap touching it, like how you would keep guacamole. I don't know if it makes a difference, but that's what I do.
I remember reading in a Japanese cookbook about the author's annual miso making exercise, so presumably at least a year.