yaaay congratulations biffle and earlgrey .. Lovely scan pictures ... 
Emma ... Mine took about 30 minutes, mainly because the baby was camera shy and wouldn't lie on its side for a proper NT measurement so the sonographer had to make me jump up and down and jiggle around to get the baby moving. The main things they do is measure the baby to check your due date (its not always the most accurate by LMP. I got moved 3 days) .. They check the fluid at the back of baby's neck to see if there is a higher risk of downs, edwards and pataus and do a quick limb and head count to make sure everything is as it should be.
Its definitely a big milestone. I was so relieved after mine. Good luck with yours next week 