If you're just using it to clean glass bottles, then I would just try to see what kind of mileage you can get out of it.
Since PBW is a cleaner and not a sanitizer, I'd just use it until it doesn't work anymore as long as only glass is involved. If it isn't cutting the mustard anymore, mix up new.)
Plain old bleach water works well for me if the goal is just cleaning off labels and bottle deposits. It's cheap, too.