Hi I have a 2005 Jaguar x350 V6 with 66000 miles it drives great but I am assuming it still has the original spark plugs when should I expect to change them

 

The V8's call for the first change at 100,000 miles.
I replaced the ones in both mine at 90+ and they appeared to be the originals.
Car still ran well with them, saw little if any real difference in performance.

 

10yrs or 100K miles, whichever is first.

I think the 10yrs might be to reflect how often the car has been started i.e. how many heat/cool cycles the plugs have endured. I suspect there's not so much a wear issue of the metal at 66K but the rest of the plug must be more likely to crumble the more heat/cool cycles.

Personally I'd change them (well, would have already done so!) but it's obviously up to you.

 

Didn't see the ten year part, but bought both used one at 85,608, the other at 97,155.
Both ran well, but figured approaching 100,000, and 12 years.
That basic maintenance was necessary soon.
I replaced the coils while there, with inexpensive Chinese.
Some of the old ones were tight, swollen on removal.