The validity period for JAR-FCL professional exams was 36 months from passing the first subject. I gather this has recently changed to 36 months from passing the last exam, that should give people a little more leeway to complete both CPL and IR.
The CPL does not limit you to 5700 Kg but it does limit you to single pilot types.
If you go outside the 36 months and have not completed the IR you will have to resit the exams. What is not clear is which ones you will have to resit. Could it be just the IR examinations? Nobody knows!