If you like what you see, you'll be auto-subscribed to the annual Premium subscription ($3.99/month but billed annually ($48)) at the end of your free trial.
If you'd prefer not to subscribe, cancel your free trial before the 7 days are up by navigating to your More page > Settings. Clicking your Subscription status will open up your Subscription settings in iTunes/Google Play. Tap on the subscription and then toggle off "Automatic Renewal" to cancel.
What happens at the end of my free trial of Glow Premium?
