It was posted before by rockitman that he was told the A90 has a 600-1000 hr stylus lifetime... I am not sure how true this is... their FAQ seems to make this general statement about all their cartridges:
Just feels too broad a statement to apply to their high end cartridges...If normal attention has been paid on cleaning record as well as stylus as mentioned, we find that 600 hours up to 1000 hours is possible without degradation of performance. However, concerning PRO systems used for "scratching" and "back cuing", we have experienced stylus lifetime to be substantially less because of their unique application. As a consequence DJ`s will have to consider about 500 hours at the most.