Sign up for a free account so that your tunnels don't time out.
There are limits for users who don't have a ngrok account: tunnels can only stay open for a fixed period of time and consume a limited amount of bandwidth.
You can always restart your tunnel to reset the limits, or you can remove the limits by signing up. After you sign up, be sure to connect your account's authtoken to the ngrok client.
It only takes a few seconds.