All I know is what I've heard. I haven't had any problems that require that solution... Yet... So I have no empirical data on how the effectiveness decreases over time.
It's like many other drugs/chemicals. If you use it constantly, your body will build up a "tolerance" for it, so it takes more of it to achieve the same effect.
Back when I was acting the fool, hanging out in bars, etc, it was no problem for me to drink a couple of pitchers of beer, ending up slightly buzzed.
Now, one beer is enough... two, sometimes, but any more than that is too much.
My system cannot handle "too much" alcohol... if I have one more than my "limit", I get sick to my stomach.... takes all the fun out of it.