The worthless answer: it varies.
A better answer: I just ate a butternut squash I bought a year ago.
After a year you should have had time to replenish your stock, so I say a storage life of a year is 100% awesome.
Last summer and fall I stocked up on garlic, sweet potatoes and winter squash when they were cheap. I bought a bunch from Fahrmeier Farms at the end of the summer, then when that stock ran low I replenished with non-local sweet potatoes and squash from the grocery store (my heart shed a tear just saying that). I am about 99% sure the second stock-up was around November.
I wrapped the sweet potatoes in newspaper and flyers (à la Root Cellaring), and put them and the winter squash in the “root cellar” section of our basement. We haven’t made any attempts to control the temperature or humidity of the basement, so these are less than textbook ideal conditions. And yet, we had success.
I think one or two sweet potatoes of the whole big pile I bought went bad. Since they were all individually wrapped, though, the bad ones stayed contained and didn’t affect any others.
Acorn squash don’t have as long of a shelf life as the rest of the winter squash, so don’t expect them to last for a year. They should be used within a few months.
My butternut squash, though, were stellar examples of natural storage. At the end of October this year I finally remembered that I still had a squash from last year, and figured I should use it up. I wasn’t sure what I would find when I opened it up, so I sliced into it with some trepidation.
(Helpful hint: to make the skin easier to slice into, soak the squash in hot water for 10 minutes. This will soften the skin and make you less afraid that you’re going to chop a finger off when you cut into it. Just dry it off before you take your knife to it so you don’t slip.)
I cut into it, and … nothing. It was a butternut squash. Slightly drier around the seeds than a squash that was not a year old, but still very edible.
Now, I’m not saying this was the finest specimen of a squash I’ve ever seen. I’m saying that it’s amazing that I can still eat something after it’s sat in my basement for a solid year.
I happily peeled the squash, chunked it up and added it to a Swiss chard chickpea slow cooker squash stew recipe from Food Network Magazine (a little guilty pleasure I allow myself only when we’re flying somewhere).
And we had a lovely dinner, even though we were eating year-old squash.