Hi
I like to say it "dependent" on how many changes that type device has..
I backup every day devices that has daily changes.
I backup every week devices that has not 1-2changes per week...
I backup 1 time devices that we don't change.
I also backup changes in HW like cisco "show mod"
I like to split the backup jobs to device type that will not overload the job ...
like backup to
"daily Cisco AS run every day at 6:00"
"daily Cisco Core run every day at 5:00"