Question

How long after you quit smoking do your lungs start to heal?

Answer 1

A body never stops healing, and so they don't "start healing" once smoking is ceased - it simply continues. Noticeable effects start within hours.

Explanation:

The human body (and, in fact, anything that is alive) is constantly repairing itself - flushing toxins, repairing damaged cells, replacing old and dead cells, etc. And so a body is healing before smoking is ceased. It's just that the active ingestion of poison has ceased and so healing is much more effective.

Inflammation and irritation of the mucus membranes of the lungs reduces within hours.

In days to weeks, your ability to take in more air improves.

There's much more here:

https://www.livescience.com/59667-quit-smoking-lungs-heal.html