20 minutes might be the case for some pre term infants (for whom you have to be particularly careful not to have in car seats too long). However for healthy full term infants, 20 minutes would be a bit on the conservative side.
To answer your question though, a healthy, full term baby should not be in a car seat for any length of time greater than 2 hours. If it's just a one off, then it's likely it wont matter as much, but on a regular basis, no more than 2 hours at a time if you can help it. Not only is there a risk of "flat head syndrome" (which can also occur with pro-longed use of things such as electric baby swings) but there is also risk of cutting off the airways from being too upright (amongst other problems).
I'd also recommend that you read these links:
http://www.mothering.com/articles/growing_child/child_health/car-seats.htmlhttp://news.bbc.co.uk/1/hi/health/6216892.stm
