If you access Headspace through Amazon Alexa, or Google Assistant, the session length will default to the meditation length used most frequently on the pack that you are currently using.
With that being said, you now have the option to choose the meditation length that you would like to do as well. In order to do a different session length, you just need to say what exercise you would like to do, and the session length of your choice.
Here are a few examples:
"Today’s Meditation for 20 mins”
“Continue for 20 mins”
“Sleep Exercise for 20 mins"