Publishing Outlook Calendars. What is the server-side setting for sync frequency?

I'm working on a custom implementation of a server (VERY basic WebDAV) that accepts Outlook's published calendars.

Outlook seems to have a setting (see the screenshot below) where it accepts a parameter form the server that tells it how often to re-publish the calendar and that's the parameter I'm trying to figure out.

If the setting is not specified Outlook defaults to 60 minutes which doesn't work for me but I cannot find any information on what the parameter might be (I am aware that Outlook's settings can be adjusted within the UI but I need it done automatically via a response from the server). When using Wireshark I see that when first publishing the calendar Outlook sends a PROPFIND method to the URL of the server:

PROPFIND /path/to/url HTTP/1.1
X-Office-Version: 15.0.4771
Depth: 1
Content-Type: text/xml
User-Agent: Microsoft Office/15.0 (Windows NT 6.3; Microsoft Outlook 15.0.4771; Pro)
Host: example.com
Content-Length: 114
Connection: Keep-Alive
Cache-Control: no-cache

<?xml version="1.0" encoding="utf-8" ?> <D:propfind xmlns:D="DAV:"><D:prop><D:resourcetype/></D:prop></D:propfind>

which then responds with an XML (I'm assuming this is where the setting would be, but cannot find any information on it):

<?xml version="1.0"?><a:multistatus xmlns:a="DAV:"/>

I'm lost at this point and not sure where else to be looking after lots of Googling and traffic sniffing (of Outlook <->Exchange communications but none that I found seem to be using the non-default param). Any suggestion where would be a good idea to look for something like this?

There is no Outlook Server setting for frequency of polling. This setting is at the client end. On an email client "check for new messages" is used to avoid overloading the server with requests. See the MS Outlook website .

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