miss83, how long your DS is depends on how co-operative baby is with moving around so the person scanning can see all the vital organs, limbs, head etc
If I am not wrong, the scan should check all head measurements, neck, hands, arms, legs, kidney, blood flow through heart, facial features incl. cleft lip. They also check to see baby's movements. ( I am looking at my scan results here haha)
My friend's said theirs took half hour to 45 mins. Mine was 20 mins because baby was super co-operative.