The budget airlines in Aus require you to be there 30 minutes before take-off.
I've missed a flight because of this; I was about five minutes late. I got on the next flight but the T&C clearly state they have no obligation to make it up in any way.
EVERYONE (not just budget airlines) wants you there 30 minutes before take-off - more if there's checked luggage involved. It takes 20 minutes to board, so it's fairly reasonable to have 10 minutes for getting through security and walking to your gate.
The only time where you might get away with arriving 20 minutes before take-off is if there's no security line and your gate's right near the entry to the departures lounge.
