Hello hazelnut9595,
I understand FaceTime is being disconnected after being idle for a certain amount of time. I'd like to give you some information about this.
Technically FaceTime calls are designed to stay connected until one of the people on the call actively ends it. There is no set amount of time that a FaceTime call will end on its own, and there is no setting to enable or disable something like this. It's possible either you one of your friends could be accidentally ending the call. It's also possible the call was ended due to a poor internet connection or a dropped internet connection.
Here's more info on using Group FaceTime: Use Group FaceTime on your iPhone, iPad, and iPod touch.
Thanks for using Apple Support Communities.
Take care.