Returns the duration, in seconds, of the item. Still footage items have a duration of 0.
• In a CompItem, the value is linked to the duration of the composition, and is read/write.
• In a FootageItem, the value is linked to the duration ofthe mainSource object, and is read-only.
Floating-point value in the range [0.0..10800.0]; read/write for a CompItem; otherwise, read-only.