I haven't had a lot of experience preparing people for exams, but have had a lot of experience being prepared for them
It can be incredibly dull if too much time is spent solely on exam pieces, and especially if a teacher leaps from one exam to the next without doing any interim stuff, as then you have to work that much harder having essentially leapt to the next grade without doing the work leading up to it.
I would agree that 6-18 months seems a long time, but it depends if someone means 18 months working just on the pieces and scales, or 18 months between exams - the former would be a cruel way to treat students (18 months on the same pieces??? ARGH!), the latter is fairly reasonable depending on the student.
I have tried with all the (very few...) students I've taught NOT to make them play stuff they hate, or if it is for a particular technical point, to only insist on them coming to grips with the "meat" of it, and not to expect them necessarily to perfect it. Usually there is an alternative if a teacher is willing to make the effort to find it.
It depends also on why the student is taking the exam and where they are in relation to the grade standard. At present I am helping a friend work towards grade 1 and teaching a student who's aiming for grade 3 in the spring. My friend is taking the exam almost solely for exam practice and confidence building, she is well beyond G1 standard, so actually working on stuff for the exam is just getting to grips with what is expected and practising weak areas: my grade 3 student is genuinely a grade 3 student, albeit, I think, a good one: therefore every area needs to be covered.
I think my G3 student is probably more typical. We do scales to some degree every week, and when she becomes confident in one octave, we add another, or we start a new scale. Aural work we do each week but not explicitly - just working at present on strengthening the areas she will need in order to cope with aural work later - as a non-pianist I hope to make use of Hofnote later to help her get the requisite practice of actual exam tests. Sight-reading we do little and often, both in terms of explicit "this is sight-reading practice", and in terms of her having pieces of music she has not seen before and being encouraged to "have a go".
Pieces: I have had her working on one exam piece consistently and a couple of other non exam pieces on and off. Once she got her head pretty much round the piece we've rested it, with the intention of returning to it later, and started a non-exam piece with a playalong CD as a bit of light relief, but at the same time I have her looking at a study which is potential exam material but also works on some areas she needs to look at, and I've also loaned her a cd with recordings of all the pieces from list A, which she hasn't done anything from, to see which ones she fancies.
As a flautist I have the disadvantage of not all the exam pieces being in the same book, but it can also be an advantage - pieces do not have to be presented as "exam material" unless I choose to present them that way, and as far as a student is concerned, unless they have gone away and read up about it, any piece I provide may or may not be exam territory.
I haven't done a lot of exam prep and I'm trying to walk the fine line between making sure she will be ready, and not wanting her to be bored of the pieces - hence my intention to make sure she has a good choice of possibles from the syllabus, and working on pieces for a while and resting them, so she has a bit of prep under her belt but is not working any one piece absolutely to death! As a student the two things I most hated were feeling underprepared or being sick of pieces (as a piano student I managed often to have both problems on the same pieces - very uncomfortable

) so I'm keen to try and avoid that scenario!
Hmm don't know if this helps at all or makes any sense, but as a novice teacher this is what I have been doing!!