How often should I change the oil on 1998 Boxster ?

[aqus]

Thanks for the info. The main reason for my hesitation is because last year I stripped the oil plug where the 8mm allen key goes in. Had to take it to the garage which they used an extractor to remove it. So now a little worried to do it and wanted to touch it least as possible...
Noticed here I am not the only one this happens to...Porsche dealer said should change the oil plug every few years as it can get a little used inside.