Interesting article, lots of information.

Anyway, I haven't printed PLA for some time (years really) and recently got a few spools for free. Newest one was 3 years old, oldest 9. Out of 6 spools only one was unusable (unopened, bambulab, i believe, it turned into oily paste), two opened spools were really dusty and first layer on the spool got brittle, underneath they were fine. Oldest was verbatim and it was perfectly fine.

I have a bunch of 6yo pla, not kept well, I'm slowly getting through. It loads and prints fine but snaps in the pipe overnight after the print finishes and everything cools. I'm planning on just unloading it after every print.

If you have a dryer give that a try. I had the same thing and it was just because it was too wet. I bought a dryer and it was just fine after that.

Yup. I don't have a tube (really should make one), filament sits on top of my enclosure and just goes straight into my extruder, and the following morning after printing old PLA there will be PLA confetti all over my print bed and a tiny bit sticking out of the hot end.

Dry it well. PLA is not super hydroscopic but over 6 years it will soak up a lot of water.

I've had regular basic pla start to get pretty brittle in as short as 1 year after opened. And others that are 3 years old and fine.

I store them all in green crates. Away from heat and light.

Funnily I have a similar PLA (maybe 8 years old), it's very brittle, but if I can get it loaded it print's fine, though a bit stringy.

I doubt it's moisture though, I had in a filament dryer for 2 days and it still behaved the same.

Brittle and stringy are hallmark signs of wet PLA. If it's sat for a long time it's likely very saturated. And some dryers take longer due to the way they are designed.

Yeah, I know, but I was in a dryer for over 48 hours at 50C , and multiple times afterwards when I was drying other spools and it never got better.

I think it's just so old and damaged that that is just how it is now.
It's from the previous owner of my printer and just sat out in the open for 7 or 8 years

Ha. My second printer is a DD ender 3 pro, I made a little clasp that holds cotton wool and clips around the filament so it cleans it as it goes into the head. It's surprising how much dirt ends up in the wool. Anyway if it snaps overnight the clasp yeets off in a random direction and is waiting for me on the print bed in the morning. As my octoprint doesn't have great night vision it being missing is the only way I can tell the filament has snapped. 🙂

I bought a dryer but I have too many filaments to keep them all in it. Once you've dried it, how long do they stay better in the open air before they go bad again?

It's completely dependent on the conditions you keep them in. If you live in a hot humid place then not long like a couple days to weeks. If you live in super dry cold areas then it's fine for years.