I use a ssh in a script that pulls the config every day.
e.g
config sys global
set admin-scp en
end
( snippet of a sample script. You would list all fw-device or you could read these in from a source file )
#!/bin/bash
#
#
#
D=`date +%G%m%d+%s`
#
#
scp admin@fw1:sys_config ~backup_user/fw1.$D.cfg
than you run it under cron. You will need to craft public-key to automate this and apply the keys under the user on the fortigates.
I' ve compile a post of other useful tricks and tips on my blog here;
http://socpuppet.blogspot.com/2013/12/fortigate-tips-tricks-from-socpuppets.html