Board Thread:Questions and Answers/@comment-5773355-20121121214931/@comment-24492510-20141103175807

From the first episode to the last episode, around 7 years and some months passed, because:

Series 1 and Series 2 take place 20 years after the end of the Great Purge. Series 1 is placed in spring, while Series 2 is placed in autumn.

The start of Series 3 yake place in late winter, 1 year later than Series 2, so it is placed 22 years after the end of the Great Purge.

Series 4 is placed 1 year later than Series 3: from November (23 years later) to April (24 years later).

Series 5 is placed 27 years after the Great Purge; it starts in winter and probably lasts 6 months.