I believe the written exams work like this:
The written examination credit comes into effect when all the written examinations have been passed in the qualifying period of 3 years and the written examination credit is valid for 10 years from the date of issue, except for aviation law where the examination pass must not be more than 5 years old as detailed in rule 61.17(c) and (d).
I believe once you pass the flight test and get issued ATPL Licence you have it for a lifetime subject to BFR's just like a CPL.
I'm 2 exams away from getting my fATPL so this stuff is only what I know so far, I could be wrong...