Coldbare
You are thinking about the issue backwards. The time a ground briefing takes is totally dependent on what you need to teach.
All ground lessons follow the same process
You teach
- Must know information, then
- Should know information, then
- Nice to know information
A properly done FI course will teach you how to teach. A big part of that will be what to teach before each lesson. This could be a lot, or nothing at all depending on what the lesson is.
Finally it should be understood that there are 3 types of ground instruction
1) Groundschool, to teach theory
2) Ground instruction, to teach how to do a particular exercise
3) Preflight briefing, mandatory before every flight and includes the Route of Flifgt, Weather and Notams, Airworthiness of the aircraft and pilots and any other safety items