Is there a way to change the refresh time with JRiver?
Not that I know of, or have seen mentioned anywhere. It really is quite frequent though. I've been surprised by how quickly it has updated in the past, as in less than five minutes on my HTPC which was running when my IP Address changed by rebooting my router.
Or know what it is ?
Only by monitoring what it currently is, and when it changes after you force an IP Address change, perhaps by rebooting your router while MC is running on the LAN, as I'm fairly sure a restart or startup of MC will do the update the Access Key data. (Confirmed)
You can check what IP Addresses the MC Access Key has stored using the URL
http://webplay.jriver.com/libraryserver/lookup?id={Your Access Key}.
Just replace the {Your Access Key} with your six character Access Key and refresh the page until you see a change.
Or if you are good at using Wireshark and can work out how MC checks the Public IP Address, you may be able to monitor that activity. Hmmm, maybe look for when MC checks the webplay.jriver.com site.
I had a quick look and the request to update the Access Key is obvious when MC is restarted, but I don't see regular checks to see if the Public IP Address has changed. So you would have to look for something other than packets being sent to the IP Address of the webplay.jriver.com site. I'll leave further investigation to you.
Gizmo is a great App, which is why I kept using it for a long time, and still use it a bit now. But I have a network issue myself because of a bad router, and Gizmo doesn't work well with that issue. Completely different to your issue. So I updated to JRemote2. It is a once-off purchase and doesn't cost a lot, and it sounds like it would get around your issue by using a Dynamic DNS service.