Hello and welcome to Apple Support Communities BBQHORSY66,
I see that you're unable to see the call duration for a Group FaceTime call. I understand your concern, I'd be happy to help.
You should be able to see this information in the Phone app under the "i", like you stated. If you're not able to see this information, force close the Phone and FaceTime app, and then restart your device. Once your iPhone starts up again, check to see if you can see the call duration.
How to force an app to close on your iPhone, iPad, or iPod touch
Restart your iPhone
If the issue remains, respond back with some more details. Which version of software are you currently using? Have you been able to see this information in the past?
I look forward to hearing back from you.
Have a great day!