realised i've been running the original borough cx tyres on my tricross for over 3 years and god knows how many miles. I've had 3 punctures in 2 days and the rear seems to have stopped being puncture resistant. On the road where there is no tread wear per se is there an amount of time after which it's a good idea to replace tyres?

All tread will eventually wear, unless the roads round you are like billiard tables.

I've had Krylions last 2-3,000 miles, my Spesh Roubaix tyres have just gone after about 1500 miles.

When they wear out

yeah they've worn but it's not like you can see the carcass yet. Just trying work out if i've been unlucky or if it's time to invest in some new tyres. Miles wise they've probably done 2000+

2000 miles wow thats good going.

On a commuter bike I find the tyres get cut up and worn and you start getting punctures more often. I generally change after the 2nd in short succession. They also tend to square off and be less round in profile, so you can see wear.