How many plays can you get out of a good record?

There are so many subjective judgments and system specific elements to your question that it is impossible to answer with any degree of statistical certainty. The first point is who is making the judgment that the record has deteriorated sonically? Each of us will have a different sensitivity to that. And then of course there is the question of what cartridge are you using, what is the vertical tracking force, what are the errors of set up, What tonearm are you using? And the fact is that the answers to those many elements that go into determining record wear will change over the lifespan of an LP. I have LPs that I have owned for 40 years and more.. During that time they have been played on many different turntables with several different tonearms and several different cartridges. All I know is when I listen to them now I can say they sound good or not so good. I try to concern myself with how the LP sounds in the here and now, with the turntable tonearm and cartridge I am using right now.

Back in the 1960’s and 1970’s, worn or badly misaligned needle/cartridges did do damage to a fair amount of the 33’s and 45’s in my collection. In particular I wonder if my meticulously set up but lousy tracking Denon 103 carved a few little nasties in my inner grooves. But in the past 30 years or so my high-end turntables/tonearms/cartridges have done essentially nothing to my precious vinyl. Sure, an occasional click or pop will show up. But gone are the days where hashy string sections or spitty sibilants would eventually make an unwelcome appearance.

I have used Last Record Preservative since it was introduced almost forty years ago. My LP's are still in like-new condition, some of them bought new and now fifty-five years old. I have also always used the best record cleaners available at any given time.

My LP's and my drum collection (all vintage, some of them dating back to the late-1920's).are my most-prized possessions. Girlfriends and wives come and go, but records (and CD's) and musical instruments are forever. ;-)

Now we have the relatively low cost technology to make transparent digital copies off vinyl re-play.  Someone should do this with some newly acquired high quality LPs, wait 10-20 years, keep track of the use, and then play the vinyl and compare it to how it sounded 10-20 years prior.  That is the only way to make a subjective judgement.  I suppose you could shrink the time element by playing the same LP over and over again and compare digital recordings done before vs after. There may be formal studies where a test LP with test tones is played repeatedly, so as to detect loss of HF or bass frequencies.  I don't have a good digital recorder, even if I cared about this issue.

A lifetime for sure. After that it's out of my hands.