when should sparkplugs be replaced?

They are supposed to last 100,000 miles. Replace them with the proper ones recommended by the manufacturer and they will also last 100,000 miles.

Thanks

I am not sure where some of these people get their information. These plugs are good for at best 90K miles. They must be iridium plugs. Use Denso or NGK.

Spark plug information is in the owner's manual and is usually at least 100,000 miles. There may be a few exceptions. Guru9FSV8, are you sure you know what you are talking about?

Copied from a Iridium Toyota parts page posted on a dealership's website. "Iridium Spark Plugs These spark plugs have many benefits: Smaller center means higher firing performance Iridium has a 1200ºF higher melting point over platinum Iridium plugs last up to 110,000 miles, longer than conventional spark plugs"

I have worked on Toyotas for 32 years and the owners manual says “at 90K or before”. During my time of being a regional mechanic, any vehicle that came in with more than 95K on the plugs always had issues with corresponding parts. Coils, wires, ignition module, distributors, rotors, severe misfires. I will stick to what the manual really states.