I don't teach it, Tess, (though I was taught it for many years) I just know that 6 months is the upper limit I could stand or would expect anyone else to stand just learning 3 pieces - on any instrument! At the higher grades sometimes you might start earlier than with lower grades, but then you'd mix it with non-exam rep. I simply wouldn't expect anyone, much less a child, to do just 3 pieces with no other input for that long! Even doing pieces for 6 months or less I would suggest that one should do other pieces too, and sight-read not just sight-reading examples but fun things.
In fact I don't think I've ever JUST studied exam pieces and been happy about it, though I seem to recall that some of my teachers worked that way (my old piano teacher for one...)

. Now that I'm largely choosing my own repertoire for flute I always have several pieces in varying stages of preparedness alongside my exam pieces, both easier and more difficult stuff, have a bunch of things to do for fun or sight-reading, and don't just learn one piece of each of the lists, and it's so much better. Towards the exam I'll concentrate more on exam material, but even then I doubt I'll completely stop playing other things. And for scales etc if they're studied alongside pieces as a habit, you never have to stop and gen up on them for months at a time.
With my piano teacher, anything that it challenging enough to me to be lasting for more than a month or so (I have lessons once a fortnight) will always be being learned alongside a few pieces that I can polish off rather more quickly. I study a mixture of pieces that are well within my abilities that I can practice playing as musically and expression-fully(!?) as I can right up to pieces which I have to really practice to get anywhere with... only once have I given up on a piece as being just simply too hard, and that was "too hard right now". When I am a bit more advanced I'll have another go because it was a lovely piece and I could manage it, just about, just couldn't play it confidently or musically! No doubt if I'd've struggled on I would have eventually learned it by rote, but I'd've hated it by then I think!
If a piece takes 6 months to learn even though it's the only thing you're doing then it's probably too hard, and you'll be bored to tears...