Are the OEM plugs iridium? If so, change 'em every 80k miles, whether they need it or not If they are not, get some iridiums and forget about it.

I used to run copper plugs and change them all the time, but it's just not worth the effort with the quality plugs and ignition systems in cars these days

I'll buy platinums if I am being cheap and/or unsure how long I will keep the car.

I know some people like them, but I have had bad experience with the Bosch +4 plugs, and Bosch plugs in general. I look for NGK (favorite) follow by Denso and Autolite.