After 60 days they will be stopped. After 90 removed
https://video2.skills-academy.com/en-us/exchange/mailbox-migration/manage-migration-batches
Migration batches with a status of Synced that have no administrator-initiated activity (for example, no administrator has stopped and restarted a migration batch or edited a migration batch) for the last 60 days will be stopped. All batches with Stopped or Failed status will be removed after 90 days. All batches with Completed status will be removed after 60 days.