Weird, I generally only get 3000 miles out of the ones on Bromptons. And I really know when they're done, they squirm about for about 100 mile before the blue appears.
Brommies, in my experience, are hard on rear tyres.
Strictly, the Schwalbe tyre for a Brompton is a Marathon, not a Marathon Plus, so it doesn't have the extra puncture protection.
Which might be why I had two punctures on mine.
I wonder if a Tannus solid tyre would be a good solution for a Brompton, on the back if not the front.