Just saw this...hopefully you got your answer by now...but if not my understanding is:
- Global sets the configuration in the policy store and therefore any Policy Server connecting to that store would use it
- Local sets the configuration in a local configuration file and takes precedence over the global
So you can set it global 24 hours which all 8 of your Policy Servers would follow. If, for some reason, you wanted one of them to use 12 hours you could set it locally on only that single Policy Server (end up with 7 every 24 hours and 1 every 12 hours).