I have also treated most of my LP's with LAST, and have been very pleased with the long-term results. I don't know that there is a magic number for how many times an LP can be played before audible signs of deterioration occur, but based on my experience, I think several hundred times is a reasonable guess
Sdcampbell, I too use Last. I have several hundred LP's that were treated many years ago and they still play as new.
This afternoon I listened to an original Decca a friend bought in Europe. It was from the 1950's, and the performance and sound were both breathtaking. Amazing that anything can remain viable, still providing excellence nearly half a century after it was manufactured.