Your Unlimited membership will expire one year from the date of your purchase.
To see the date of your Unlimited expiry, go to your Account page. Click on the 'Unlimited' heading to see when it expires:
As your membership period comes to an end, you'll see reminders around the site prompting you to renew. Follow those to renew your membership or click the 'Renew now' button in your account area (shown above).