This value, in minutes, is the configured or actual length of the collection window for one specific, already-completed 15-minute collection interval, retained in a rolling history buffer so a series of past intervals can be graphed. On this element it is what you divide the accompanying access-hit byte and frame counters by to turn a raw cumulative count into a meaningful average rate for that completed 15-minute interval, and it is also how you spot a truncated bin -- one whose Duration reads less than the table's normal interval because a clear or restart cut it short. For a normal History bin this should read 15; a value of 6 instead would mean the access-hit counters accumulated for only 6 of the expected 15 minutes, so any total should be scaled up before comparing it against a full bin.