Domains are valid for 1 year from date of sign-up. They are automatically set to renew unless you cancel or otherwise specify at the time of your purchase.
You can always check the status of your Custom Domain and adjust the auto-renewal in your "My Domains" page via your Account's page. The custom domain info will only show to the admin that purchased the custom domain.
For domains purchased directly via support, If you want to renew it past the one a year we recommend adding a note to your calendar reminding you to contact us a week or so before the domain expires because the domain won't renew automatically.