How does one know if their body has healed from a smoking addiction?

How does one know if their body 
has healed from a smoking addiction?
How long does it take? Does it happen; 
or at some point is it just too late?
I did two tests recently to see if I had, 
in fact, recovered from an almost
four decade addiction to tobacco.
While these tests were not scientific,
they still pointed out the difference
between quitting completely and 
the respiratory issues one faces by 
continuing to smoke.

First, I used to be the lead singer 
in a band that played Desperado 
by the Eagles and Wicked World 
by Chris Isaak. I always struggled 
with hitting the high notes of those 
songs, sometimes trashing them 
completely. Recently I reattempted 
these songs, sailing right up to those 
notes and nailing them more cleanly
than I ever did while I was smoking
and singing with the band...

...then yesterday, I attempted to swim 
two lengths underwater- a distance 
of 90 feet. While I smoked, I was 
barely able to do a single length. 
This time, I did the test with enough 
air to make the second turn. This feat 
is some sort of accomplishment, 
especially for someone who, less 
than four years ago, was experiencing 
respiratory failure.  I'm breathing a 
whole lot easier now. Please share 
this story with friends, as there was 
a time that I never would have 
believed that quitting would change
my health the way it did. Just quit.
It doesn't hurt. Get addicted to the 
taste of fresh air. It's fantastic...