Please let me help you with just two suggestions.
One is the word you mentioned in your post: duration.
In formal written English it is a commonly used term meaning a period of time throughout which something lasts; I've come across it many times in contract agreements (duration meaning the period of validity of a given contract).
When referring to the period of activity of an organ or person of authority the expression 'term of office' is often used (e.g. Chairperson's or Supervisory Board's term of office.)
Hope this helps.
All the best,