Im guessing around the 20,000 klm mark ..that's what a few of mine have looked like before I got'em changed. Mainly when travelling ... that last bit of tread disappears real quick I've found.
The bike has 18k+ miles, had this one on it when I got it in 2012, so maybe its the 2nd one? I noticed the stripe on the right with 450 miles left on my trip, luckily made it home.