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:
[Image descriptor: the 'Account' page, accessible from the site's right hand menu. The heading, in black, says 'Account overview'. There are six subheadings, and Unlimited is the second from the left after 'Subscriptions'. Further down the page, the subscription expiry date is shown above a pink 'Renew now' button with the current Unlimited price next to it.]
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).