But miles ridden is
related to total engine revolutions. It's a perfectly linear relationship as there are gear ratios and idle times to consider, but for our rough calculations, it's pretty damn close. And much easier to track, as all bikes are required to have working odometers, and folks are trained to think in miles.
Hours run, as they do in dirt bikes, takes idle time into consideration, but has no bearing on engine load in different gears.
So if you really wanted to be picky about it, you'd track both.
And then you'd do the math, make a few assumptions, round a few figures, and come up with a reasonable service interval that takes all of this into account.
Which, I imagine, BMW has already done when they came up with their recommended 6,000-mile service interval.
My last bike had a waterproof calculator mounted on the handlebars. That was more for quick time/speed/distance calculations, and occasionally fuel consumption, but it did tend to gather some odd looks from random bystanders.