500 GB is way more space than you need for ftp backups, it's likely that each of your backup files will be under 2 GB. I will also note that ISE does not keep track of past backups file ages, and it does not care if you backup file is older than 30 days. You set a schedule for ISE to run backups such as every week on friday night, or daily at 2 am, and it will run and export the tar.gz backup file. It will be the responsibility of the backup server to maintain and remove the old backup files. If you only want to keep 2 back up files, you need to write a script to clean up the ftp folder. ISE will not perform any clean up of your repository, it only creates backups files, or reads them to restore, it never deletes a backup file.
The backup file contains the ISE admin node config, so they are relatively compact. If you ever have to restore it, you deploy a VM from an OVA or ISO and point the ISE node to the ftp repository to select a restore file. It's important to backup the certificate store separately because this is not part of the scheduled backup file you get.