Not JAR but it might help give you an idea:
I had 3700 hrs when I converted to a UK ATPL in '99, just prior to JAR implementation. I did two theory courses for the exams of 6 & 8 weeks + exam week after each, and under 10 hrs flying over 1 1/2 weeks including the required CPL & IR flight tests. I started the 1st theory course in June & finished the last exam around Oct. As said, the flying took less than a fortnight.
JAR has requirements about how much theory & flight instruction you will have to do. Theory course time can be variable depending on what allowances the theory school will give you. You will have to do a min. of 15 (?) hrs for the IR conversion + possibly some amount for the CPL licence test. It might be possible to include prep. for the CPL test in your IR prep.