Never timed it, but I know it has a mind of its own, depending on temperature (but you still need to press the button yourself, yes indeed. I do it all the time to defrost/fog my mirrors and sometimes it shuts down real quick, sometimes it seems to last forever. And no, the mirrors do not start heating by themselves below 5 degrees, like I read elsewhere).
--
(2003 Silver 1.4 Tdi Exclusive(SE), 95bhp/235Nm chiptuned by ABT (finally!), black interior, sport seats, heated mirrors, JVC KD-SH909RB cd-receiver, Infinity Kappa 63.1 + Vifa on-dash tweeters, 12" Subwoofer on 300w Amp)