I use Conti GP4000s too. I usually get 2500-3000 miles from them. I will replace a bit before the wear indicators are gone. Worn tires seem to get more flats. I had always changed front and back at the same time, but am now experimenting with not doing that.
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'96 Giant ATX 760 MTB
'01 Bianchi Eros
'05 Giant OCR Llimited Carbon Fiber + upgrades