When to change a damaged tyre is one of the most divisive questions in cycling.
One camp will ride a tyre until the canvas is showing, and even then continue until the canvas looks thin!*
The other camp will replace at the first sign of damage. **
I tend to the latter camp. But I am sure that I've also binned tyres that had several thousand miles of useful wear in them. The thing is, I'm not expert enough to look at a split and say for definite that all will be well and anyway, I'd rather ride than spend time at the roadside repairing a tyre or puncture.
* This is a very slight exaggeration!
**This isn't an exaggeration.