You'll get a duration if Data Detectors detects a date interval. For example with this string: "from December 10th at 2 to December 12th at 1"
and this logging code:
NSLog(@"Match: %@ (duration %g)", result, result.duration);
I get:
Match: <NSDateCheckingResult: 0x7fc2da810830>{0, 45}{2014-12-10 13:00:00 +0000-2014-12-12 12:00:00 +0000} (duration 169200)
(I'm in the UTC+1 timezone, this is why the hours are offset in the result)