Thanks for reaching here! If you have purrchased App Service certificate (A private certificate that's managed by Azure. It combines the simplicity of automated certificate management and the flexibility of renewal and export options.)
To elaborate App Service Certificates are issued for 1 year. You may toggle the automatic renewal setting of your App Service certificate at any time, select the certificate in the App Service Certificates page, then click Auto Renew Settings in the left navigation.
Select On or Off and click Save.
Certificates can start automatically renewing 31 days before expiration if you have automatic renewal turned on.
Further there is also an option of creating a free App Service managed certificate (A private certificate that's free of charge and easy to use if you just need to secure your custom domain in App Service.) this is fully managed by App Service and renewed continuously and automatically in six-month increments, 45 days before expiration.
You create the certificate and bind it to a custom domain, and let App Service do the rest. On side note free certificate comes with limitations: