This Unsigned32 (1..65535) field states the configured length, in minutes, of this measurement bin. Because this row is a closed, completed interval from the PTP performance-monitoring history table for this timing session, 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.