 You don't need to be an 'investor' to invest in Singletrack: 6 days left: 95% of target - Find out more
.... how big/deep/many before you replace it?
Can you see the inner tube yet?
Nope.... There's about 4 cuts in a Durano S, the biggest about 4mm long, the others about 3mm...
done 4000 miles on mine and think they might need replacing but i always think just one more week.
at some stage they give in and i'll have a long walk home
Its not the distance Andy, I'm unfamiliar with road bikes and therefore tyre strength / limits, so I just wondered about the "limit" about which I need to consider changing the tyre.
Doesn't sound too bad. You'll always get surface cuts, you just don't want them going too deep.
If they are just little cuts in rubber don't worry. Pick any little bits of glass out of them to stop them working deeper and causing you a puncture, if you're really keen put a drop of super glue in the crack to re-seal.
Another thumbs-up for a small amount of super glue applied to a tyre slash, worked everytime for me. Go for the normal stuff instead of gel unless you're trying to stick down a loose flap of tyre.
The pic below is showing a cut I received to some Gatorskins from a sharp flinty bit of trail I decided to ride...
[img] 
[/img]
I've been riding it like that since May last year without issues and that includes a little bit of off-road. IME most little nicks, cuts and most slashes can be repaired with the above method or simply ridden until they fail.
Edit : It's actually deeper than the photo suggests.
I go round with a blob of 'puncture repair outfit' glue.
And if it's a bad cut in an otherwise alright tyre a puncture repair patch on the inside.
I just replaced my rear GP 4 Seasons because I was down I the carcass, could see the 'cross hatched' stuff all the way around, full of holes, no flats though! They're pretty resilient!
My last conti commuter tyre flatted all the time when it got worn, interesting.