what is the normal exhalation and inhalation duration(in seconds respectively) in a single breath for a average adult

The length of the breathing depends on the characteristics of the person and of what he is doing. We cannot talk of ‘normal duration in seconds of exhatlation and inhalation.’  A breathing reference for a given person could be the rythm of breathing during a calm healthy sleep in an appropriate environment and temperature with which we could then compare his other forms of breathing.

I have learnt the wispered ah as part of the Alexander Technique. I personally find it very useful, at least when done properly. However, you state that no breathing exercices are part of the Alexander Technique. Is the whispered ah part of the AT? Can breathing exercises help to free our neck and Co.?

I will try to clarify for you what I mean when I say that we don’t teach breathing exercises in the Alexander Technique. But before that: yes the whispered ah is taught in the Alexander Technique. Is there any contradiction there? I don’t think so because of the difference of aim between what we call commonly breathing exercises and the whispered ah. Usually breathing exercises are done to improve breathing. People expect from them direct results. I don’t say that this apply for breathing exercises as taught within certain traditional or esoteric techniques where the pupils follow a long and different path than what you find usually in classes in the West.
You don’t start with the whispered ah. The whispered ah is a procedure in the Alexander Technique which is taught when the pupil is using himself sufficently well as to be able to start with the whispered ah without loosing his overall coordination, which includes already an improved breathing. Then yes the whispered ah will help improve the breathing even more, but more than anything else it will help getting rid of the preconveived ideas about breathing and allow in its turn to give even better directions to oneself and improve the use of the neck and head,and so on. I hope my answer is clear for you. Regards.
See also Thinking Aloud, by Walter Carrington, chapters on Breathing and whispered ahs.