Well, what technique is the teacher using?
Back when I taught Special Ed (I was a sixth grade teacher, so—yeah, about the right age group) we had reasonably good luck with the Wilson program. Basically, it focused on trying to teach the kids the patterns by which words are formed. F'rinstance, if you have a word like "bad"—one vowel in the middle—you know you're going to get a short A sound. If you know that's the pattern, you also know how to read words like fad, cat, let, etc, and you can even figure out nonsense words like tish and hox. Of course, then you have to learn the exceptions to the rules—having an R at the end of a word changes just about everything, having a double L changes a, o, and u but not generally i and e . . . anyhow, it gets more and more complicated but it's all about the logic of spelling. It does exist, really.
The problem is, in a medieval setting, there's no guarantee that your teacher will approach the problem methodically. He/she might just say, "C-A-T spells cat. Memorize it and we'll move to the next one." Which is much, much less efficient. I mean, yes, when you read you will end up with a lot of words memorized, but that's not the optimum place to start.
I also have to admit, I have never taught a twelve-year-old non-reader to read. I did get a boy once who hadn't learned his alphabet, but when we retested him his IQ was around 58 or so and he got moved to the Comprehensive Development Class. (I taught Resource, which was designed for people with learning disabilities and assorted oddball problems, not people with developmental delays or mental retardation.) Our kids generally read at first or second grade level; they'd gotten there on rote memorization and blind stubbornness and then stalled because, for one reason or another, they never really got reading. They tended to improve by a bit less than a grade level a year, but many of them were already convinced that they were stupid and reading was stupid and Resource was a giant waste of time. And, of course, they were still fighting past whatever learning disability they had in the first place.
If you had a reasonably bright, highly motivated illiterate student of twelve or so I bet he/she could learn to read well enough in two or three years. It depends a bit on what you need him/her to be able to read, though. A light novel (which wouldn't exist in the culture you describe, anyway) is a lot different from leafing through an ancient, possibly mis-copied religious text full of metaphor and symbolism. You can read a lot of things with a sixth grade reading level, but if you don't have a lot of books to practice on . . . I don't know.
Of course, all bets are completely off if the written or educated language of the time is different from the spoken one, like Latin.
Izunya