When do you replace a tyre?

Similar to Judge dreadful. When they simply can't be ridden anymore because canvas is showing through or there's bad bulges. I tend to get a pretty good run out of Conti 4000's, probably 1500 - 2000 miles for simple wear, though the sides tend to cut up before that. And unless you run your tyres very hard and corner very hard you won't notice the effect of squaring off.

I find that once the TWI is gone on the conti's you are about 3/4 way through the wear

I replace them when they get cut up, they start to perish or any bulges appear, usually. That’s about 20 miles on a pair of Continental GP4000sII / GP4 seasons, usually. More robust tyres tend to last me at least a few months, and I’ve had a years worth of riding out of my last pair of Tannus Airless tyres.

I get 6month (around 2500miles) out of gatarskin rear tyre before I start getting punchers in quick succession.

I used to replace front and back at same time but now I often move the rear to the front after 6 months and put a new one on the rear. Never made it passed the wear indicators before they are too cut up.

Like this

http://217.199.187.65/conti-tyres.co.uk/tread-wear-indicators-explained/

I use Conti GP4000s - they seem to have a material with a criss-cross pattern in it backing on to the rubber. When I see the imprint of that layer, I know that the tyre is near its end. I've never ever noticed the effect of the squaring off. Maybe I don't corner fast enough....

Clearly, the less rubber there is, the less the tyre is resistant to punctures so if road debris is a factor around where you are, that may be too late.