I had 126,000 miles on my first set of plugs, and just replaced that set at 119,000 miles. Never noticed a change in startup, fuel mileage or idle smoothness. But seat of the pants feel seemed to indicate wide open throttle performance was better with the new plugs.

I'm drive 25,000 miles a year though, with very few short trips.

2002 X5 3.0 245,600 miles
2004 325i 109,000 miles