Hello Snow,

Import set scheduled cleanup is a scheduled job that runs everyday at midnight. By default, it will query all import sets that are older than 7 days and delete the import set and all related tables, eg. the import set table records. If this scheduled job is not active on your instance, we recommend that you activate it as soon as possible to mitigate the growing import set tables that you may have, especially if they are recurring imports.

Import Set Scheduled Cleanup - ServiceNow Wiki