CalDAV (Calendaring Extensions to WebDAV, documented in RFC-4791) uses the iCalendar (Internet Calendaring and Scheduling Core Object Specification, documented in RFC-5545, not to be confused with Apple’s iCal) format for the data exchange. iCalendar accommodates non-standard properties that start with a “X-” prefix.
X-PUBLISHED-TTL is the property that maps to the recommended update interval for subscription to the calendar. It’s supported by Microsoft for Outlook & Sharepoint, and possibly by some more calendar publishers, but not by Google or Apple.
Example values:
X-PUBLISHED-TTL:PT1H (every hour)
X-PUBLISHED-TTL:PT120M (every 120 minutes)
There’s also some work-in-progress to officially add a similar property to the iCalendar spec. According to the latest version of the New Properties for iCalendar draft proposal, the new REFRESH-INTERVAL property would be used in the following way:
REFRESH-INTERVAL;VALUE=DURATION:P1W