You can find information about backups here: Backup and Restore
You can see your current backup scheme using zmschedulebackup -q.
The f line indicates the full backups, the i the incrementals, and the d is for the deletion.
For the time out before the deletion of the backups, you can specify either in days (d), months (m), or years (y).
The command to change it will resemble something like this:
The number and letter between the d and the quotation marks (7d in the example) will specify how long before the backups will be deleted. So in your case you may want something like 2m for two months, or 1y for one year, etc.
zmschedulebackup -R f 0 1 * * 7 i 0 1 * * 1-6 d 7d 0 0 * * *
Note that you will need to put in the full and incremental data even if you're just changing the deletion option.
And don't worry if you lose the default config, it's as easy as typing zmschedulebackup -D to get it back.