Does the time to replace tire(s) differ from whether you talk about 23 or 25 vs much bigger sizes? Smaller tires need to run at higher pressure and wear makes them more sensitive to blowout. And the risk is higher since if you have 23 size tires, you are more likely to ride in more sporting, riskier (read faster) fashion.
I ride my 23s well past when they develop flat in the middle, I watch for cracked rubber on sidewalls when it gets more serious (due to age I suppose since I don't ride high mileages and tires last me several years). Once I asked about my concerns with the sidewalls cracking in LBS and was told it is OK, just the top layer is separating and as long as I don't see threads...
Last tire change came about by not being able to avoid fresh pothole at high speed which resulted in four snake bites, two on each wheel
and while the tires seemed OK, I was offered nice ones when I went to buy couple new tubes and decided it was time to replace both tubes and tires. Later on I patched those tubes and kept them as spares.