I usually start to see electrode degradation around 10-15K..which for most isn't much of a concern.. For some cars leaving the plugs in for 20-25K isn't an issue.
On my personal cars and for the clients of mine that are as picky as I am I like to drop in fresh plugs about the 10-15K mark..nice crisp edges on the electrode etc, etc..
I'm working on getting a Reliable source for the Beru copper core plugs...hopefully that will come through sooner rather then later.
As an aside, I did just pick up 2 sets of Beru copper core X (4 electrode plugs) for my 560's...hope to try a set sometime soon..I'll report what I find after I get some miles racked up
Jonathan