In normal situations it takes over 100 years to decompose it.
Still much better than non bio
Long term, yes, but I'm not sure many landowners will be best pleased to hear that it'll only be their great-great-*-grandchildren that see the difference on their site.
The intardtubes confirms that PLA will only biodegrade before the next ice age if you're actively composting it, which we are not.
Refs:
Research study commissioned by the California Department of Resources Recycling and Recovery and produced by California State University in Chico:
http://www.calrecycle.ca.gov/publications/Documents/1435\20121435.pdf
Long story short: PLA doesn’t degrade under any measurable criteria in 365 days in a simulated marine environment, complete with sand, microbiota, 30 deg. C, etc.
For PLA degradation in landfill environments:
http://www.material.chula.ac.th/Journal/v18-2-2/83-87 RUDEEKIT.pdf
Not much degradation there either.
A generic biodegradable polymer review article that includes a short paragraph about PLA and acknowledges (somewhat by omission) that it is only biodegradable in industrial composting facilities (by and large missing in any major US city I’ve lived in):
http://homepages.rpi.edu/~grossr/_doc/publication/SCIENCE 2002 297 803.pdf
Another, highly cited one:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2769161/pdf/ijms-10-03722.pdf (skip to this one first if you are a science geek and want to have a bit of perspective on plastics; not all plastics are equally bad, and PLA is by far NOT the most biodegradable one.)
Scientific American thinks that a PLA bottle will biodegrade, yes, but it will take between one hundred to one thousand years, in a landfill environment:
https://www.scientificamerican.com/article/environmental-impact-of-corn-based-plastics/
And all that assumes that your "bio" BBs are actually made from PLA. Bear in mind that Chinese baby formula manufacturers have repeatedly poisoned babies for profit, with the collusion of the local Party officials. Do you think they care about mislabelling ABS BBs and marking them up?
https://www.reuters.com/article/us-china-milk/probe-finds-20-percent-of-china-milk-companies-in-scandal-idUSPEK27908420080916