It really depends on one's experience level. An inexperienced pilot should expect to take longer to convert than an experienced pilot. If the pilot is also learning something new eg adding a first time multi, or multi-IR then expect even longer.
My conversion to a UK ATPL was generally easy - but then I had 3700 hours, Australian & FAA ATPLs and was a current IR/multi instructor. In my case it took 9.5 hours from first flight to end of a combined CPL (well, ATPL it would be for me, but same test) & IR flight test. No sim. training that I recall.
Exams weren't that bad. My worst mistake was focusing too much on the harder Nav group items and not quite enough on Air Law & Radio. End result I failed those two by a single question each and had to resit them. Bugger. No time lost though because I re-sat them the same week I sat the Engine group.
I realise that I did a UK national licence and not the current JAR however any school with a decent reputation is well versed in JAR specifics by now. Anyway my intent was to try to show that there can be great variability in time to convert depending on prior experience, even allowing for the JAR 15hour IR training requirement.
Something that might still be possible: I was also able to save quite a bit of cash with the Cl. 1 medical by having all (or nearly all) medical procedures & investigations done in Australia and then bringing the results & reports with me to Gatwick. The CAA's fee was for a Cl.1 renewal even though I'd never held a CAA medical before. This option might still be available if you phone & ask about it.