Do Acids/Bases undergo dilution/bio-degradation when exposed to air? I spilled some cleaning fluid/drain opener in my trash can, a long time ago(almost 3 months ago), I didn't tried to clean it up. The cleaning fluid was diluted, and I spilled a small amount not more than 30 ml, I think.

The bottle does not say, what the cleaning fluid contains, but it probably has Sodium Hydroxide(if it is an alkali-based cleaner) or Hydrochloric acid(if it is an acid-based cleaner).

Now, what I have heard about Sodium Hydroxide, is that it reacts with carbon dioxide in the atmosphere, and lose its corrosiveness over time.

So, is it possible that NaOH got completely carboxylated in 3 months and nothing corrosive remains?

Now, about strong acid such as Hydrochloric acid, since they are hygroscopic, do they become dilute over time?
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.