New spark plugs should last at least 40 000 miles and the wires should last around 65 000 miles. Even though it might not be necessary, i still change my plugs and wires every18 months. I installed the thicker focus sport wires so they should last me a good while now.
I would just check the plugs once a year and good plugs should last a long while, I would think anywhere from 30,000 to 80,000 miles if the motor is in good condition. As far as plug wire I would let them go until I have a prob. with them or see cracking.