when conti TCK 80 are done?

[wildeone]

I have rode them already for ~5500 miles (9k km) and still they don't look too bad to me.
I will post pic of rear tyre later on but what is the best way to determine knobby tyres status?
When knobs are gone completely? Maybe somebody has pic tck 800 when decided to change?

 

[2brnut2b]

I'm not 100% sure but I think there is a marker somewhere...I changed mines at about 8500km not because they were done but they were cupping and very noisy.