Filament Moisture Exposure Timer
Some filaments tolerate open air for weeks; others start absorbing enough moisture to matter within hours.
Why the safe window varies so much by material
How hygroscopic (moisture-attracting) a filament's base polymer is drives almost all of this difference. Nylon is extremely hygroscopic and can absorb enough moisture to cause problems within just a few hours in humid conditions; PLA is comparatively tolerant and can often sit out for a week or more before it's worth worrying about, though humid climates shorten every one of these windows.
Symptoms of printing wet filament
Popping or crackling sounds at the nozzle, steam-like wisps visible near the hotend, a rougher or bubbly surface finish, and reduced layer strength are the classic signs — the trapped moisture flashes to steam inside the melted plastic and disrupts the extrusion. Nylon in particular can look and print almost fine while actually being significantly weaker than dry filament.
Drying doesn't need to be complicated
A dedicated filament dryer is the most convenient option, but a kitchen or food dehydrator, or an oven held at a low, material-appropriate temperature with the door propped slightly open, both work for most materials. Check a specific drying temperature and time for your exact material before using heat, since too hot can deform the spool or the filament itself.