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.
i opened a similar thread in scooby forum, to share and post the evo we spotted in town, see whether the owner is in da house......share some interest and chats......