Do Acids/Bases undergo dilution/bio-degradation when exposed to air?
What you heard about NaOH is right: it surely got completely carboxylated in 3 months, maybe even in 3 days. Not that the resulting compounds are completely neutral to the environment, but yes, they lack that corrosive ability.

As to the more general question of what happens to inorganic substances once they are out in the open: dilution is quite possible, biodegradation is not so likely. Some of them, like hydrochloric acid (and most other acids), would pretty soon end up reacting with some random compounds out there to yield relatively harmless products which would become normal part of the environment. Others (think of heavy metals) may have long-lasting adverse effects.