The Backup is an additional service connected to the MSSql Database, therefore it will expire at the same time, even if purchased at a different time.

The renewal of the Backup service can be made during the 
renewal process of the Database to which it is connected, from the website:

The renewal of the Database can be made starting from 90 days before the date of expiration and, as with all the services, Aruba shall send notices by email to the contact email address, to remind that a failure to renew will result in the cancellation of the service.

If the Backup service is not renewed, the procedure of the relevant 
MSSQL Database shall be followed and, after 45 days following the date of expiration without renewal, it will no longer be possible to renew the Backup service but only purchase it again as a new service, with no possibility to recover the details saved in the previous backups.