since i document every job including drains, i already have a head start on the history of the drain.
i give a warranty that i feel good about. meaning there is no set in stone warranty period. if i feel it should have lasted 6 months, i'll come back and re-clean it. sometimes i charge just for the additional time like an extension to the original job minus any machine charge.
by tracking the prior history, i know whats worked in the past.
once i know there is a serious issue i document that and let them know that there is no way to guarantee a line that has issues that a snake, jetter can't fix.
as far as going back to jobs that i felt plugged too soon, it's only a few times a year.