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 specific bin belongs to the completed-interval history table, its value is frozen the moment the interval closes, so re-polling it later will return the same number -- useful for pinpointing exactly which past 15 minutes a problem occurred in. 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.