The JAA FI rating should be the same in any JAA State but its not. To cover 125 hours theoretical training and 30 hours flight training will take a minimum of 19 working days, typically thats 4 weeks. In most cases add another week for contingencies and you have a realistic minimum time.
If you are offered a course in less time then something must be left out. To be able to instruct you must know how to brief, that means understanding as well as presenting the brief. Look at the schools briefing material and match it to other well established material.
There is good and bad in everything, everyone wants a good instructor, nobody wants a bad one. As the method of instructing varies from State to State there is merit in obtaining the qualification in the State where you want to use it or you may have to learn it all again.