Freek,
your 3-6 months is correct as it has taken these 2 extremes depending on sim and instructor availability.
In our case we started in the last week of Sept and are programmed to do our last sim (of about 40) on 5 Jan but this date already looks like blowing out by a week or thereabouts, so 3 1/2 to 4 months for our course.
The classic course is the longest as it also includes a Flight Engineer conversion. S/Os also relieve the F/E, but you probably already knew that.
If the statement above about 4 classic PUITs on the November course is correct then expect another 4 in the new year. There are also some F/Os being trained on the classic at present.