Spark plug wires should last 100,000 miles or more.
But it's recommended that you change the spark plug wires at the time you change your spark plugs or after every other spark plug change.
I personally just replace the spark plug wires along with the spark plugs when I change them because it's just as easy to replace the spark plug wires than just changing the spark plugs.
Having both new Spark Plugs and New Spark Plug Wires can help improve gas mileage and improve horsepower and engine performance.
Sometimes it can be hard to remove the old spark plug wires from the spark plugs and you have to cut them off anyway so I just replace the spark plug wires regardless if they're brittle or not.