I know that heating wet filament in the oven is the best way to dry it, and that desiccant is typically used to keep filament dry, but will it also eventually dry a spool that has been sitting out in a humid environment? If so, how long would this typically take?
For example, I have a 1kg spool of tightly-wound PLA sitting out at 55% RH for months. If I put it in an airtight container with sufficient desiccant, will it eventually become dried? And how long would it take?
I used to dry shrooms in a tote with a bucket of damp rid, it's not exactly the same but they get dry real fast. I suppose it would work for filament, too.
No, it will not. It will help protect it from ambient moisture, but it won't remove moisture already within the filament, since the filament itself acts as a desiccant.
I'm sure a dehydrator would be faster and drier, but I've used these to take a roll from printing poorly to printing acceptably in a few days. Just make sure it's freshly recharged and you put them together in a small airtight container.
I use a modified food dehydrator. It’s cheaper than a filament dryer and does the same thing. You can often find them used for a good price. From my experience desiccant alone is not enough to dry out a filament.