(not sure what the required duration is), The required duration ( length of the inverted line at the top AND corresponding upright line at the bottom to finish the maneuver) is set by the size of your 1/2 loop which will in turn be set by the length of the beginning upright line past center to start the maneuver.
The reason I say this is because the double immelman is a square box with rounded ends(the 1/2 loops), NOT a rectangle Sooo
the distance you fly past center before you start to pull the 1/2 loop will determine your overall size. The 1/2 loop will need to be twice as tall as that line was long - then the inverted flight across the top will need to be as long as the 1/2 loop was tall.
MOST beginning flyers ( I know cause I did it) always make the top line too long and make a rectangle out of it. Quite often this is a symptom of a sport plane that lacks on vertical power to make the 1/2 loop tall enough.

Sorry this is so long winded - lots of words to describe something simple -but it's important as most ALL sportsman pilots miss this until it's pointed out to them. Check it out in the aresti
http://www.flyinggiants.com/forums/a...4&d=1159250815

Eddie