The threshold for excessive backup history in the Quick Scan Report is greater than 1 year of backup history, or greater than 10,000 rows in the backup history.
SELECT MIN(backup_start_date) AS first_backup, COUNT(*) AS total_rows FROM msdb.dbo.backupset WITH (NOLOCK);
For those who are running frequent transaction log backups, the backup history can grow pretty quick.
To see the backup history:
SELECT * FROM msdb.dbo.backupset WITH (NOLOCK);