I can’t tell you that, all I can say is it was open on a shelf since my Tiko died in 2017, I think. Who needs sparkle cyan filament? It printed fine after drying. My basement is not very dry at the best of times.
A few tips, if you don’t live in Nevada keep filament in a sealed bag. It’s a pain but I never have had to toss a roll.
RH is a fickle measurement. It is a misleading. Relative humidity is relative to temperature. The RH will drop in a heated environment because it is heated, it has nothing to do with water evaporation.
Real example. My work room was 17.8ºC when I got up, Cold. RH was 43%. I turned on the heater and it is now up to 21ºC the RH dropped to 35.6. Good right?
The amount of water in the air is 6.5gm/m2 for both, the absolute humidity is constant. Warm air can hold more water. At -35ºC 100% RH and 1% RH are almost exactly the same the air cannot hold any water vapour.
This is why I use fresh desiccant every time and leave it in the dryer for 24 hours at a minimum. Measurements inside the dryer need to be converted.
I Always dry PC, and Nylon 100% of the time. PETG if it is open a while I will put a few feet in an old Monoprice and just extrude some if it pops into the dryer it goes. PLA gets a bend test and if it breaks in it goes.
My exact technique is when PLA breaks (I pull 4-5 feet off the roll and test there) I toss the first few wraps and loosen the roll up. I dry it for a couple of days. My current dryer is all automatic (daft thing really) a hex. I liked the Sunlu V1 better until it had an accident. (gravity)
I have not had issues with printing the old filament, ever. I don’t doubt there is a change, is it enough you can’t overcome? No.
I have bought rolls of filament that were brittle straight from the shipper. Some filament suppliers buy pellets and they sit out in huge boxes for an undisclosed length of time. The rolls ship brittle.
One of the reasons why new people, I always recommend starting with premium brands, like Prusa, or Polyalcomy. They control everything carefully. The tell is if the rolls are really neat, all aligned perfectly. It is rare for the filament out of the box to have any issues.
I don’t suggest always using crazy expensive filaments, once you get used to the printer it isn’t necessary. Low cost filament on occasion comes bad. Figuring out how to use it could be helpful especially when you have 10 or 15 rolls arrive all brittle.
Just trying to help.