If you use Office 365 and have associated your public domain name (also known as custom domain in Office 365/Azure AD), a new coming feature is going to help you maintaining your associated domains.

This feature, called Expiring Domain Insight, will notify you with an high-severity alert, sent to the tenant administrators, letting you know every 90, 60 and 30 days when a domain is expiring (below a sample of the alert)

Then you can click the View alert details to get more insight about the expiring domain