How to ask siri about event length in calendar?
As far as I know (I am on iOS 9.0.2), Siri cannot explicitly tell you the duration of the event. For events that are longer than three (3) hours, it also does not provide the end time of the event. Siri also (mostly) cannot do things related to previous items requested if you do not refer to it clearly, which is why "how long does it take" or "how long is it" will confuse Siri.

You can only work out the duration of the event if they are or shorter than 3 hours as Siri will only show you the start and end time. Past 3 hours, you have to physically enter your calendar app to find out.