How long it takes you to learn the pieces depends on a number of things:
1) How well you want to be able to play the piece when it's finished (just to pass, distinction, better than Alfred Brendel (I wish
) etc).
2) How good you are relative to the exam when you take it, for example I've been playing for just under four years now and did grade 4 a few months ago. I could, should I have wished to, have been playing for four years and only just done grade one (in which case the pieces would have been relatively a lot easier to learn); or I could have chosen to do grade 8 instead of grade 4 now (in which case it would have taken me a year just to learn three pieces and not much else and been considerably more difficult).
As you get higher up the mark schemes each extra mark gets harder and harder to get: the differnce between 23 and 24 might be to even out a passage of quavers in one bar which is technical work and although it requires time and effort is just a matter of practise. The differnce between 28 and 29 might be to make your interpretation really convincing and enjoyable to the examiner, this might mean going through the piece considering the weighting on every note, the feeling to be conveyed by every phrase etc. and will take a lot longer. It's also subjective and some people seem naturally better than others at communicating, it takes time to develop this skill and it's not something that one can just learn by doing some exercises it has to develop over time.
I wil usually spend a good few months on my exam pieces but will be able to play them to a passable standard after one month (the last exam being grade 4). I then spend a lot of time getting everything as perfect as possible, it is this that takes most of the time in preparing a piece so if what was required is to pass then a month to be prepared but to aim for the highest marks a lot longer: it's never ending! I am one of those people who like to over prepare for exams (not necessarily a good idea) and will leave plenty of time and prepare the pieces to way over the standard needed in the hope that despite inevitable nerves I will still get high marks. Now between grades 4 and 5 I am working on grade 5 and 6 repertoire so that when I do get the grade 5 pieces I will be able to do them well because I'm used to playing the harder pieces (I've always done this for exams) you could argue that I'm taking grade 5 when I should be doing grade 6 or whatever but this is just the way I like to use the exams: to be comfortably within my abilities rather than to be pushing what I can do.
As for how the time taken to learn pieces changes when you get higher up, logically I guess if you're going at the right speed perhaps each grade should be equally hard to you at the time of doing it than the previous ones were (otherwise you should have more experience before attempting it). However I'm not sure that this is necessarily the case in practice just because as the grades get higher and closer to fulfiling the potential of the person doing them it may get harder to progress further. Or maybe if someone started the grades a little earlier than perhaps they were ready but then slows down later (or just catches up with themself so to speak) they will find the grades easier as they go up.
Maybe at grade 8 people wil find playing the pieces technically easier but it will still take them longer to complete them because they need to put in so much more artistically which takes time. An example, my boyfriend has recently done a diploma playing the Pathetique sonata, when he first picked up the piece he sightread it through such that the untrained ear wouldn't really notice there was anything wrong with it (a performance with which the average grade 1 pupil would be delighted with if it was a the standard of a grade 1 piece); he then spent at least two years working on both technical issues and artistic interpretation of the piece. My (non-musical) mother couldn't tell that it was any different to when he started playing it (fortunately for his sanity I, his piano teacher and two ABRSM examiners could! (not that the examiners heard the first rendition but I'm sure they would have noticed if they had!)).
So basically it depends if you're equally on top of each exam when you come to tackling it and you want equal marks then they'll be equally difficult. If the exams are getting harder and harder they will continue to do so until one takes some more time out to get more on top of them before proceeding to take the next one. If they're getting easier and easier, and one is getting marks that are high enough for one to be happy with then it's worth considering going faster.... One thing that I would guess does switch as one gets higher up the grades though is the ratio betwen the amount of time taken physically getting to grips with the technical side of the piece and being able to get through it; to the time spent on interpretation of the piece and artistry.
. Maybe this reflects more on my age than the difficulty of pieces (I restarted last year after a 15ish year break - I was a teenager when I did the early grades!).
). Basically I cover quite a few pieces around just the level that I can sightread but not with instruction on how to play them, just playing them through a couple of times, mainly as a sightreading exercise, and I leave polishing or slightly polishing other pieces to a time when there are no exam pieces to polish. 
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