There is no option for auto-renewal. When you start to approach your expiration date, you will start getting email notifications that you site is about to expire (these will start a month before it expires).
If you fail to renew before the expiration date, your site will expire, but will remain pending for 14 days. If you renew before that 14-day grace period expires, your site will instantly restore itself.
If you fail to renew by the 14th day, the site will delete itself, and renewal will require someone to manually restore your data (which, depending on how long you've waited, could take a bit of time).
When you have your site, you'll notice that at the top of the Admin page is a constant countdown informing you of how much time remains on your site, and you can renew at any time.
Finally, just as a reminder, we currently bill by the year only, which means that you'll only need to renew once a year.
