.duration()

Gets the timeline's duration or, if used as a setter, adjusts the timeline's timeScale to fit it within the specified duration. duration() is identical to totalDuration() except for TimelineMax instances that have a non-zero repeat in which case totalDuration includes repeats and repeatDelays whereas duration doesn't. For example, if a TimelineMax instance has a duration of 2 and a repeat of 3, its totalDuration would be 8 (one standard play plus 3 repeats equals 4 total cycles).

Due to the fact that a timeline's duration is dictated by its contents, using this method as a setter will simply cause the timeScale to be adjusted to fit the current contents into the specified duration, but the duration value itself will remain unchanged. For example, if there are 20-seconds worth of tweens in the timeline and you do myTimeline.duration(10), the timeScale would be changed to 2. If you checked the duration again immediately after that, it would still return 20 because technically that is how long all the child tweens/timelines are but upon playback the speed would be doubled because of the timeScale.

This method serves as both a getter and setter. Omitting the parameter returns the current value (getter), whereas defining the parameter sets the value (setter) and returns the instance itself for easier chaining, like myAnimation.duration(2).play(1);

var currentDuration = myAnimation.duration(); //gets current duration
 myAnimation.duration( 10 ); //adjusts the timeScale of myAnimation so that it fits into exactly 10 seconds on its parent timeline