You can check the remaining time on your current billing cycle of Core Membership from the Billing and Payments page, which can be accessed from the avatar menu in the site navigation.

Please note that only your current billing cycle is represented on that page. If you have a pre-existing Core Membership, either from gifts or past sales, they may not display accordingly. If you have questions or concerns with regard to your Core Membership, please contact DeviantArt Customer Support.