Managed mine in just over 15 hours (5 of which were sim). Benefits were doing it at LBA (LFS) so no positioning for the skills test, and also I went straight through the FAA to JAA CPL/Multi conversion on the exact same aircraft before commencing the IR conversion (so no time wasted getting used to the a/c type, etc..).
Wasn't bad going to say that I was 2 years out of practice from my FAA IR pass. Treat the 15 hours as your minimum but bank on spending a bit more. Can be done.
Oh and watch out for the con artist schools (naming no names but they are big 'reputable' ones) who will insist on telling you you need 40 hours minimum to convert FAA IR > JAA IR.
