Depends on who I'm talking with.
For work, the conversations last normally just a few minutes. Maybe 2-10 on average. Depends upon what we're talking about. Its a huge plus as I can show the other person what I'm talking about. Not having to describe what I'm seeing, just showing them, really helps speed things up tremendously.
Here's an example of without FaceTime:
"no, not that network cable..the blue one. What do you mean there are 37 blue cables? Ok just unplug the blue net cable thats connected to the 24 port switch. What? There are 24 blue cables in the switch?"
"Ok, see that blue cable on the left? Yeah, that one. Unplug it. Ok, done"
With family, it ranges from who I'm talking with. If its my wife, its normally just a short call since we car pool to work and see each other all the time anyway. But with my mother, its longer since I only see her maybe once a year. Say about 30 minutes.