When should the plug wires be changed on a 99 Forester?

 

My owners manual and maintanence book does not say.

There is no service interval. I usually change mine every 60k miles, as they tend to harden and crack due to heat and exposure.

I just changed the wires on my 99 legacy. It looked like it had the original wires from 99 on it and I thought it ran perfectly. I put new NGK wires from the stealership on it and I think the car reclaimed about 10hp and is also now smoother through the full range. Long story short: if you've got orginal wires it's definately time to change them.