There is no specific relationship between the theoretical knowledge exams for a MEP Class Rating and the issue of that rating. The time limits relate to the Licensing Skill Test.
"The type rating course, including theoretical knowledge, shall be completed within the 6 months preceding the skill test." [JAR-FCL 1.240(a)(4)]
"Each applicable item in the appropriate skill test shall be satisfactorily completed within the six months immediately preceding the date of receipt of the application for the rating." [JAR-FCL 1.240(b)(2)
So, you have 6 months from the first day of the approved course to pass all applicable items of the LST and the application must be received within 6 months of passing the first applicable item of the LST.