If you access Headspace through Amazon Alexa, it will automatically default to a 10 minute session. You are currently unable to change the sessions length. Apologies for the trouble!
However, for Google Assistant, the session length will default to the meditation length used most frequently on the pack that you are currently using.
You also have the option to choose the meditation length for the Google Assistant. 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 minutes”
“Continue for 20 minutes”
“Sleep Exercise for 20 minutes"
Need more help? Email us at firstname.lastname@example.org.