The 20 weeks scan (5 months) is when the doc who's doing the scan (can't spell the right name lol) measures the baby, everything, head, arms, legs, body, umbelical cord, brain, heart, lungs, kidneys and other vital body organs. Bascially at 20 weeks, the baby is big enough to see if he/she has everything and has no initial problems, holes or any disforms that can be viewed. This is an essential scan to have. So if you LO is good and stays still it can take 15 - 20 minutes, however, if your LO is a wriggle, expect to be there for 30-60 mis or even do half that day and the rest of the scan the next day.
You may seen women go in, 10 mins later come out, walking about, eating mars bars and hen going back in. This is so the baby changes position.