Like all seeds/nuts walnuts will go rancid over time because of oxidation of their oils. Walnuts seem to go faster than, say, almonds. So if you buy a bag and aren't going to use them in a week or two after opening, freeze them. They'll last a lot longer that way, but obviously not 'forever.'
My experience with frozen/cool walnuts -- *warm* walnuts have nuance and depth of flavor. There's a sweetness to them. You can sort of taste some of the fat / oils, giving them a smoothness in your mouth. Cool/cold nuts may be crunchy, but I find that when they're cold/chilled the main flavor is 'bitterness' ... they have to warm up before they 'taste good' again.
That reminds me that I need to buy more walnuts!
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