It is a gut feeling I suppose,weather,it will never be in the last twenty minutes of a session,must have shown to adapt quickly to the conditions on the day,not just parrot fashon,or a point where the student may be becoming tired. the thing is , it isnt important, i was 22hrs, now a BA captain,my instructor 24 hrs, now a IRI/TRI with britsih midland ,if the weather suits instrument work or cross country then its all part of the course! dont get hung up on it!