This can be done under the Configuration > Firewall > Service Policy Rules pane. You'll want to add a new service policy and follow the wizard's prompts to match only your backup traffic (choose "Source and Destination IP Address (uses ACL) for Step 2 - Traffic Classification Criteria).
On the CLI, it will look like the following:
access-list backup-acl permit tcp host
match access-list backup-acl
set connection timeout idle
This will only apply the idle timer to the traffic matched by the ACL.
Hope that helps.