Adding to L3B's answer, the more talented one is, the sooner it's all achieved. Obviously practice time, quality of teacher, finances, also feature strongly. As a guideline, I said one exam per year, depending on results, of course was fair, but it's only a vague guideline. Everyone has different amounts of talent, time, motivation, etc., etc. So, if it's a spur you need, it could be said that you're in need of some extra work, but I'm not here to judge you! As the other answer states, I've had players pass grade 8 after 4 or 5 years and others who never got there, all from the same starting point.
It will also depend on how frequent/long your lessons are, whether you are young with 'loads of spare time', a 'married with kids, working man', you probably get the picture! Matter of fact, there's no yardstick, it's rather like asking 'how old will I be when I reach 5'2" height?...
Possibly a better question might be 'am I allowed to procure enjoyment out of the exam system?' Now, there's a question...