There is no one size fits all answer for this, but generally speaking, if you are practicing adequately, and in the way your teacher has instructed, you can expect it to take about a year to prepare for each grade level.
Keep in mind there are one or two prep levels (depending on your instrument) that must be completed before a student is ready for level one. You can think of this as musical kindergarten. Just like a student needs certain skills before being grade 1 ready for school, the same applies to RCM exams. You don’t have to do the prep level exams, but you absolutely need to develop the skills. Depending on the age of the student and the practice habits, I’ve seen a prep level take anywhere from 6 months to 3 years or longer. It really does depend heavily on your practice habits.
I occasionally get students who’s parents push from them to take exams before they are ready. To the parents, it looks like they can do it, and, in the early levels, it is often actually possible to get through the notes of a piece of music faster than the 1 year mark. Some teachers allow this, or, possibly cave to pressure from students or their parents.
Here is the problem with this approach. If you are only focusing on the notes, and only focusing on the selection of music required to pass a level, there are many aspects you are leaving out, and while a student might be able to pass the lower levels like this, by the time they get to the intermediate levels, they won’t have the skills required to play the material, let alone pass the exams. Also, this approach is a bit like if you gave a student all the exam answers to every test they would take over the year, had them shakily memorize just the answers, regurgitate them for a test, then call that passing the grade. There is a lot that goes on in a year of school that is not on any test, but that students absolutely need for their development.
NOTES are only one aspect of music. First, the notes have to be correct. Then, posture, fingering, and form also have to be correct. This is not about appearance. If you are not using correct form, you will not be able to execute advanced techniques. Think about trying to type quickly using the hunt and peck method, or signing your name with a pen grasped in your fist. There are reasons for holding things a certain way and they have to do with the needed control and dexterity.
Also, beyond the notes, are aspects like dynamics, articulation, and phrasing, to name a few, that not only are necessary for beautiful playing but also will be expected in the upper grades. If you don’t spend time in the early levels setting the student up for these, and beginning to incorporate them, they will get to the point where they are playing high level material at a very low level. It’s a bit like saying a grade 3 student is reading at a grade 8 level when they can’t understand, or even pronounce, the words they are reading, let alone do it in a way that they can comprehend the sentence they just stumbled through.
Below is what is required in level 1 piano, taken from the RCM piano syllabus. It is important to note that the technical tests must be memorized, or the student receives a mark of zero. The repertoire is supposed be memorized, and there are marks for memory awarded, though the student will not receive a zero if memorization is not accomplished.
Again, keep in mind that a solid music education that creates a proficient player will include more material than is needed for an exam and
For “musicianship tests” , the student will be asked to identify intervals and chords that the examiner plays. They will be asked to listen to, and then play back, a short melody. They will be asked to listen to a melody and clap back the rhythm, and they will be asked to sight read.
All of this takes time to learn. In my experience, if you are making it though a level in less than a year, it means that crucial skills and material are being omitted and that remedial work with be required in the higher levels.