Agree, 1 minute interval is nonsense ..
1. you are going to overload LDAP with periodical queries.
2. once the LDAP starts to have load issues and start to react slowly, your queries stockpile and will just increase the load with non-responded queries queued.
3. once there will be issue connecting to that LDAP (DoS / network outage / load on server), then FortiGate will start to stockpile non-responded queries in queue which is memory, so you will possibly overload FortiGate as well.
4. seriously, how often do you change group membership of the users and how fast you need to sync this "promotion" through all connected systems?
You just need to find a balance between sync time and for how long you can keep possibly older group membership info, versus some reasonable auto-update interval.
Besides that, if group cache record times out, then on next login spotted by FSSO there will be fresh LDAP group membership query anyway.
In summary, I do not see any reason to knock on LDAP's door extremely often.
Short intervals like 30 minutes are fine. And even longer update periods like few hours might be OK as well, as said it depends mainly on frequency of your group membership changes.
Tom xSilver, planet Earth, over and out!