In order to check your renewal date, you can click your avatar on the top right corner of the screen, then click on account settings and then on the second tab: Purchases. There you can also check your Gravit Designer PRO subscription status. (Or you can check the PRO badge on the Welcome screen. You can check it by going to Menu > Help > Show welcome screen.)
Please notice that your Gravit Designer PRO subscription auto-renews each year, so it needs to be canceled explicitly. In order to cancel your subscription, please click on your profile picture, select "Account settings", go to "Purchases" and select "Cancel" for the subscription. After confirming it, you get an email and the subscription runs out at the end of the mentioned period. No follow-up costs are involved.