As far as I know, there are no wear indicators moulded in the Metzler carcass. When it starts getting preceptibly flat in the middle, it's time. If you still have visible tread grooves you still have time left.
Gunshots proved a long time ago that the carcass is pretty thin even new. I've got well over 6 on mine and it shows no wear but then I don't ride the bike hard. I'm expecting at least 10K and possible more.
At that time, I'll most likely change out to a car tire and radius the edges.