Set ExpectedDuration based on DueDate when ProcessInstance starts

TimeSpan timespan = (DateTime.Now - DueDate);
K2.ProcessInstance.ExpectedDuration = Math.Abs(timespan.TotalMinutes);

Update:  After it's set, the expected duration remained 0 in the ViewFlow 4.6.6 for the Process but the number is there in ProcInst table.

Calculation DueDate based on duration from StartDate

System.TimeSpan duration = new System.TimeSpan(0, 0, K2.ProcessInstance.ExpectedDuration, 0);

DueDate = StartDate.Add(duration);

or

SELECT startdate, DATEADD(minute, expectedDuration, startDate) DueDate
  FROM MK2Server].]Server].]ProcInst]