I changed mine at around 75k. Longevity depends mainly on your driving, If you stand on your clutch on stoplights, enjoy frequent burnouts, high rpm take-off and such really put a number on your cluch....as far as cost, check with Adam, he posts here and works for Midamerica who carries plenty of porsche parts, when I replaced mine with a Sachs kit It cost me around $380.00 for the kit and about the same in labor...very important, when you get your job done have them check your flywheel, mine was rattling like crazy and I also had to replace it....God Luck!
