Look in LASORS, Section G1.3; it's all there!
Particularly ... "The holder of a valid JAR-FCL CPL(A)I/R with ATPL theory credits will be credited the ATPL theoretical knowledge requirements for a period of 7 years from the last validity date of the IR(A) entered in the CPL"
So, if you are flying and keepung your IR valid the exams will last forever. If you let the IR expire and do not renew it for 7 years then back to groundschool you go!
Happy landings
3 Point