We exclusively use LT tanks and have only had to replace the one we use for Rigid resin. All of the others have lasted months of storage. The rigid resin appears to swell the film which creates ripples or waves in the surface of the tank. I believe the Rigid has a different solvent than the others which is why it only appears with that resin (just my guess). Unfortunately, there is no way for me to accurately look at how many liters have been used on my various tanks, Dashboard should be able to track that but it doesn’t.
3 prints in same spot on LT tank should be no problem, very odd that you are getting that result.
In the end, replace when things stop printing to your satisfaction. I think failure mode typically ends up being mechanical damage from scraping for most LT tanks.