This Unsigned32 (1..65535) field states the configured length, in minutes, of this measurement bin. Because this is the currently open, still-accumulating measurement interval that keeps climbing until its boundary is reached and it rolls off into the history table, Duration is the denominator you need to turn any of the accompanying event or error counters into a rate (events per minute) for comparison against other bins of a different length. For example, a Duration of 15 on a History row means the counters in that row cover exactly a quarter hour, so 900 dropped frames in that row works out to roughly one drop per second on average.