Mwah, around 2 years for the drives in my main machine which isn't a lot.
My oldest harddrive is from 1982, its a 10Mb Seagate ST-412 it has had 15 years of 8-to-5 office work under its belt and it is still working fine, can't beat those old MFM drives.
Besides the 10Mb drive I have collected a whole heap of Seagate ST-225's they all work as if they're new, only one needed a low level format a while ago, it literally lost track of it tracks since stepper motor drives have no clue whatsoever about where the hell those tracks are located, 2548 steps thataway? Its caused by thermal expansion and contraction, the BIG enemy of all stepper drive, the later voice coil actuator drives all have a calibration and tracking mechanisms so the drive can always find the tracks no matter the temperature, stepper drives just step in or out the required number of times and hope for the best...