Once every 13.8 billion years.

“Beholding the Big Bang” by Arthur Ganson is a work (video below) that has a series of gears that (exponentially?) slow down as you progress through them. The first gears visibly move, but by the time you get to the final gear (the 25th-ish), the slowness with which it progresses is 1 revolution per 13.7 billion years, or, the estimated age of the universe.

This final gear is set in concrete.

“the motor drives a series gears designed to reduce its input speed such that it will take 13.7 billion years to turn the final gear in the train, embedded in a concrete block, once. “

Some quick arithmetic:

Let’s say the universe is 12 billion years old (rounding down from 13.7 for the sake of easier numbers).

That means we can think of that final gear as a clock with 12 hours. If we skip down to the second hand, we can divide 12 billion by 60 seconds to calculate how long it would take for the gear to turn the distance of one second on a clock.

Your answer 200 million years.

If we divide the 12 billion years by 360 degrees (there are so many in a circle) we find that the amount of time it will take for the final gear to turn one degree is 33 million years.

To turn a half a degree? 16 million years.

To turn 1/100th of a degree? = 330 thousand years.

To turn 1/1000th of a degree? = 33,000 years.

To turn 1/10,000th of a degree? = 3,300 years.

To turn 1/100,000th of a degree? 330 years.

To turn 1/1,000,000th of a degree? 33 years.

And that, my friends, is why the final gear can be set in concrete.