How long do you guys let PBW go? When I'm done with bottles, whether they're my homebrew or commercial, I give them a quick rinse and set aside. When enough have accumulated, I will fill one of those home center 5 gallon buckets with PBW solution and I will submerge as many bottles in there that I can (usually about a dozen). The next day I'll pull them out, drain them back into the bucket, then do a few twists with a bottle brush, rinse them with a bottle jet, then put another batch of bottles in the bucket. If the bottles were commercial, then the labels come off real easy this way. Is there a feel for how long PBW is good? There doesn't seem to be a quick test to tell, like the pH test for StarSan. After a while, especially if there were a lot of commercial bottles with all the dissolved glue from the labels, the water gets pretty murky, but I'll gladly keep using it as long as I am confident that it is doing its job. Right now I'm verifying its efficacy by visual inspection of the cleanliness of the bottle.
Another interesting thing I've noticed is that if I have a bottle that is only partially submerged, then right at the water line something starts to accumulate, like a mineral scale, and it does not want to wash off. Thinking it might be some kind of caustic scale, I've soaked the bottles in a StarSan solution hoping it would dissolve, but it doesn't. I have to use some kind of abrasive scrubber and really work to get it off; if it ends up around the bottle lip, then I usually just recycle the bottle instead of spending the effort needed to get it off. Any ideas what that deposit is and how to easily remove it? Is it PBW coming back out of solution?