a standard spark plug should last 10-20K KM...those better/performance type should last 30-50KM....
Spark plug is wear and tear part...
Also life span of spark plug depend on your engine condition...if your engine makan engine oil / wrong tuning then your spark plug will wear out faster.
Just got back from Santa Pod today. Officialy was a Supra gathering, but myself, Jurgen and crew gatecrashed to see what Jurgen's newly modified 35 would do. Jurgen also has a really big following in the supra world...