Office 365 Groups (including Microsoft Teams) automatically expire after 365 days unless a group owner confirms they are in use.

Before expiration (at 30, 15, and 1 day prior), group owners will receive a message like the following from "Microsoft Online Services Team" <msonlineservicesteam@microsoftonline.com>:

Clicking the link labeled "Renew Group" will open a browser tab.  Successful renewal of the group for another 365 days will display the following message: