How can I get the Duration of an Item via GetItem?
In essence you cannot retrieve this directly. For Items that have not closed, the difference between the EndTime and the StartTime will give you the Duration. If the item ends normally then this calculation will still work. If, on the other hand, the item ends early then the original Duration will not be determinable.
Stores items that are GoodTillCanceled will show a StartTime that reflects the original starting time and an EndTime within the next 30 days. GTC items are effectively 30 days items that auto-relist (with the sole exception that the StartTime remains at the original start time.