Apparently according to Jico (manufacturer of the highly regarded SAS stylus), the amount of playing time where a stylus will maintain its specified level of distortion at 15kHz is as follows:

Spherical / Conical - 150hrs
Elliptical - 250hrs
Shibata/Line contact - 400hrs
SAS/MicroRidge - 500hrs
This is not to say that at 500 hrs a SAS stylus is "worn out" - but at that stage the wear has reached the point where distortion at 15kHz surpasses the level specified by Jico for a new stylus. (Which I believe is 3%).

According to JICO, at 400 hours a Shibata-tipped stylus—as found on the Ortofon Cadenza Black—is worn to the point where distortion at 15kHz is out of spec relative to a new stylus. To put that into perspective, 8 hours a week of record playing is just over 400 cumulative hours per year.

At 650 to 700 hours the worn diamond tip may begin to wear record grooves at an accelerated rate, and for that reason, a good rule of thumb that I will adopt is to check this type of stylus by at least 500 hours of use, and retip it if necessary.