Data retention information for Microsoft Defender for Office 365
By default, data across different features is retained for a maximum of 30 days. However, for some of the features, you can specify the retention period based on policy. See the following table for the different retention periods for each feature.
Defender for Office 365 Plan 1
* Alert metadata details (Microsoft Defender for Office alerts)
90 days.
Note: By default, the alerts queue in the Microsoft 365 Defender portal displays the new and in progress alerts from the last 30 days. The most recent alert is at the top of the list so you can see it first.

Reference:
https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/mdo-data-retention