In a KZ or FA I’d change them every weekend or every day. Not because I have to, but because I like to have a clean plug to read from.
If I’m not worried about reading the plug I’ll use them for a lonnnng time. The EIX are made from very durable metal so they don’t wear very fast at all. Even in the motors I have that would turn 19k+RPM.
There’s also the EV and EG which are cheaper to buy but don’t as long due to different metallurgy. Then again I’ve never worn one out perse either. If I have an ev or eg knocking around I’ll use it for practice. Save the EIX for racing.
Really I only change because I feel bad, or I’m looking to read the plug.
8 seems kinda hot to me for a piped KT. But it’ll stay clean on slow laps if that’s a concern.
I’ll caution you and say I’m not very well up on the specifics of the KT though.