Mine had 42k in four years. They needed changed, but not excessivly worn.
I have run standard copper plugs in other vehicles that saw approx 30K or so a year. No problem getting 60k out of them.
If you are driving 60K a year mostly highway, I would expect you could go 80k or 90k without much trouble. But I would pull them out now if you have not already, take a look and see how worn they are. If they still look okay, figure on running the replacements farther.
Keep in mind, your emissions warranty covers the catalytic converters ($$$) for 8 years I believe. If you were to have a plug stop firing, and damage the cats with unburned fuel, Toyota would flip you the bird unless you can prove the plugs were changed at the specified interval. I would also be worried about gas washing the cylinder walls and hurting the engine if a plug stops firing.
If I were you I would pull them out now and see what you have. Make your decision based on what you see.