This is the length of this SONET/SDH port bin, an Unsigned32 value expressed in minutes (range 1..65535), per the MIB's description. Duration is what turns the Line/Section counters (Cvl/Esl/Sesl/Cvs/Ess/Sess, etc.) sitting in this same row into a rate an admin can compare across bins of different lengths, rather than raw counts that mean different things depending on the window; because this bin is the still-open, currently-accumulating interval, its value is only a partial count until the bin's Duration elapses and it rolls into history -- polling it twice a few minutes apart is the fastest way to catch a fault building up in near real time. For this OCn port bin family the value would typically read 15 minutes, so an admin dividing this row's Esl by 900 seconds gets the exact fraction of the window the Line signal spent in error.