Closed Partition Backup question

Our legal department has been very generous with regards to expiry.  Therefore, we have several closed partitons.  We have been backing them up weekly over the weekend.  Over the past month or so the back ups have taken longer and longer.

I'm looking for recommendations on the best option:

1.  Make one last back up of the closed partitions and store it permenantly.

2. Back up  closed partions once a year.

3. Backup closed partitons every six months.

4. Backup closed partitions quarterly.

5. Back up closed partitons once a month.

Thanks in advance for your input.

Rick