I havn't changed the plugs in mine for about three years now (though I don't put many miles on it). When I drove it as a DD, I would do them when the gas milage would dip down, running rough, or every 2 years what ever came first. Wires I do if the plugs looked ok but ran off. You can do a OHMs test on the wires (specs are in the repair manual if I remember right). I have done Champion and NGK and I think the NGK's are doing better in it.
If you're in doubt about the plugs, pull one out and see what it looks like. You can get a gapper cheap and then you can see what the gap is and see how close to .044 it is. If they are way off, time for plugs. Sometimes junk can build up on the plug and "weld" the tip to the electrode.