STI’s warranty covers the belt, sprockets and glide plates up to 500k cars so you can be sure they are expecting the actual life to be higher.
A belt site will drive more volume at a higher average ticket price as compared to a chain. It can also process more cars per hour during peak...
Yes it is a fuel site but they are not actively manning the wash during normal operation. At night there is usually only one person in the C-store so they can’t leave to attend the wash very easily.
Without a dirt extraction system I would recommend to inspect & measure wear on belt, sprockets and glide plates every 25k cars. With a good dirt extraction system, you can push that to about every 100k cars.