BMW says for a 128i that the spark plugs should last 100K miles.
For a 135i they say 45K miles.
For a 1M they say 30K miles.
I think that most people should change their spark plugs at ~30K or so. Basically if your car is tuned... change the plugs.
The problem with long life spark plugs is, sure they might last 100K miles(in a NA motor) but who wants to risk removing those seized out plugs at that mileage.
IF your into performance driving, you like a snappy throttle response... and power... change your spark plugs more frequently. Just like engine oil. BMW wants you to buy a new car every few years just like a cell phone. I prefer to hold onto my cars longer than two years.
Dackel