It's not quite a simple as saying the conversion course is 15 hours flying and 30 hours ground training. The requirement is currently contained in JAR-FCL 2.320(f), which states:
An applicant for a FI(H) rating .... who has been issued a specific authorisation in accordance with Appendix 1 to JAR-FCL 2.305, complies with the requirements of JAR-FCL 2.320G and hold[s] a JAR-FCL licence shall have fulfilled the requirements for the issue of a FI(H) rating, subject to the initial restrictions set out in JAR–FCL 2.320B.
The relevant part of Appendix 1 to JAR-FCL 2.305 states:
1 (a) Instructors seeking to instruct for a JAR–FCL licence including class and instrument ratings shall:.... (iii) have completed in accordance with JAR–FCL the approved relevant course(s) of theoretical instruction and flight training. The course may be modified, as approved by the Authority, taking into account the previous training and the experience of the applicant, but shall comprise at least 30 hours of ground instruction and 15 hours of dual flight instruction performed by a flight instructor holding a JAR–FCL licence and rating in accordance with JAR–FCL 2.330(f);
So, what should happen is that a CFI completes a course of training, approved by the Authority, the length of which depends upon his previous experience but must not be less than 15/30. On completion of the course and the Skill Test, he is issued with a specific authorisation in accordance with Appendix 1 to JAR-FCL 2.305. This authorisation is then exchanged for a JAA FI rating when a JAA licence is obtained. However, note that the rating should be subject to the initial restrictions set out in JAR-FCL 2.320B (i.e. a restricted FI rating should be issued irrespective of the applicant's previous experience).
However, as usual, the UK CAA makes up its own rules or, more accurately, adopts the rules made up by the major FTOs, and we end up with a de-facto 15/30 course irrespective of previous experience.
The equivalent provisions under EASA are contained in the Personnel Licensing Regulation of which Part-FCL is an annexe. The relevant information is in Article 7, which states:
Applicants for Part–FCL licences and associated ratings or certificates already holding at least an equivalent licence issued in accordance with ICAO Annex 1 by a third country shall meet all the requirements of Part–FCL, except that the requirements of course duration, number of lessons and specific training hours may be reduced. The credit given to the applicant shall be determined by the competent authority of the Member State to which the pilot applies on the basis of a recommendation from an approved training organisation.
This says much the same thing, without referring to the 15/30 minimum. However, the CAA will not be able to be so free with their interpretation and the course length for each applicant will have to be assessed separately, based on his previous experience and a pre-entry flight test, the latter being a Part-FCL requirement.