I kept running on the same original chain and casette for 4 years with occasional cleaning and lubing. Some times once every 3-6 months, others time more often. My shifting got poor lately so I finally replaced the chain and casette with a brand new set. It did not fix the problem, feels exactly the same (I suspect it has to do with cable housing or cable so I will replace those next). So as far as I'm concerned, you should not change casette or chain until either one breaks or you have extraordinary difficulties with shifting, chain skipping etc. I'm probably going back to my old set now and keeping this new set as spare.

As for rulers and chain wear indicators, I highly doubt that they really work. Chain wear is so microscopical that its practically impossible to measure. There are so many factors involved here that could make measurements seemingly go on either way. Just ride until you can't ride anymore and buy a new set of chain and casette around every 5-7 years.