Hi Coldbear,
Firstly, I think we need to understand what you mean by 'CFI'.
In the United States, under FAA rules, it means 'Certified Flight Instructor', a specific qualification with a proper definition.
In the UK, it generally means 'Chief Flying Instructor'. As far as I am aware, we don't currently have a definition of such in the UK, or indeed in EASA. Part of the role traditionally carried out at flying schools by Chief Flying Instructors is now done by the 'Head of Training' at Approved Training Organizations. I don't know if it has ever meant anything in Denmark.
Now, your questions. In England, we have a phrase 'how long is a piece if string'? Meaning, any length you like! As you say, it can be anything from 1 hour to 5 mins, either can be equally appropriate or inappropriate. For instance, if the flight exercise is a repeat of one flown earlier in the day, then 5 mins might be just right, any longer might bore the student!
Many students will ask theory questions at completely random times, as the thought pops in to their heads. If it is relevant at the time and appropriate to answer then, I would give an answer. If, however, they ask 'what is a semi-monocoque fuselage' on short final, I'd say 'we'll discuss that later!' I would then make a note on my pad to make sure that I did in fact answer the question, most important to not give them the impression that their questions don't matter.
Anyhow, it sounds as if you're planning on becoming a flying instructor. Good luck and I hope you enjoy it.
Cheers,
The Odd One