How often does Zimbra poll an external calendar for changes?

[opusmyhero]

Hello:

We have created calendars (campus events) that users can subscribe to. The question we are running into is:

How often does Zimbra poll/check an external calendar (http:// . . . . . . .ics) for changes? updates?

Based on Zimbra's schedule (assuming there is a default?), we would like to programmatically (cf/oracle) update the calendars to make sure they contain the most current information.

Please advise.

Thank you.
m