Openfire XMPP Ping Request - how to change Ping frequensy

Hello

In Openfire 3.8.2 / Client Connection Settings i have the following setup:

Disconnect clients after they have been idle for 300 seconds

Send an XMPP Ping request to idle clients.

The problem is that openfire sends XMPP Ping every 15 seconds, which is an overhead IMHO.

I Can't find where & how to change this parameter, any ideas?

Thanks in advance