Speaking from experience, it appears that you can, as I did, do the rating and test in 3 hours if the TRTO has CAA approval to do so.
I had an R22 rating on my JAA CPL(H) and needed an R44 rating and ended up doing did 1.9 hours refresher training ( I had a current,valid R44 rating on a overseas CPL) and a 1.1 hour skills test.