If you have it installed in an aircraft then you have to abide by overhaul schedules. There are some differences if the aircraft is operated for hire or not.

If you want to run it in your car (which, as tcar states, is not really a great idea), you can run 'em as long as want.

Tucker put Franklin air cooled flat six aircraft motors in his cars but other than that, I don't know of any one else who's tried it on a large scale.

thanks!

they are <15 minutes away from me. gotta check them out.

If you want a cheap air cooled flat 6, you can buy a Chevrolet Corvair! They came with or without turbo! The only problem is that they rotate the opposite direction, but with a 901 or 915 gear box you can flip the differential.