This is a common misconception. PLA is technically compostable industrially, but it will be very slow to break down naturally if you just leave it outside. It can take about 80 years, which is a lot better than other plastics, but it’s a bit misleading that it’s called biodegradable. There are true biodegradable filaments like PHA, but they’re a lot less common.
That being said, 3D printing is actually really useful, and not a significant part of the plastic waste problem.
I think the 80 year figure was for PLA, as PET lasts well into the 100s of years (450 is the number I’ve seen)
This article claims 80 years https://3dinsider.com/is-pla-biodegradable/ but I haven’t taken the time to do proper research about this.
It wouldn’t surprise me if the science on this is divided, especially because conditions (and the shape of the object) vary so much. It seems very plausible that a thin injection molded PLA cup could break down really fast in a sunny pond in a warm climate, but a thick 3D printed object buried in the ground somewhere with a cold clinate could take many decades.
Anyway, it sounds like it’s maybe not as bad as I thought, but it’s still not good to treat it like typical biodegradable materials. Especially when pla microplastics are still harmful despite being shorter lived.