Well , I study with ABRSM, and jumped from 5 - 7, I will then hope to do grade 8 in 9months,
which will build up to 2years in total.
~ from 5-8 doesnt mean you will have to dedicate that whole
time to playing the syllabus pieces either, but work on other pieces by those same composers,
Beethoven/Mozart, etc, look into their lives, who inspired them, what their lives were like.
Learning all these things should help you to understand their composing style, the emotions they went through, thus making it easier to interpret when wonder how music should feel.
Just be careful! 5-8 is a huge jump mentally and physically, ~ playing grade 8 should be more than playing what the syllabus tells you, eg, you could spend the whole 2years just learning 3 pieces.. by 3 composers, but you can miss out on so many other technical aspects..
Eg.. you could play Mozart and Chopin to a grade 8 standard.. but when it comes to bach you may only feel a grade 4-5 standard, ~ therefore learn a well rounded repertoire at grade8 standard, or build up to it at least! Whether you aim to be a performer or teacher learning about composers and playing styles its vital to havea thorough understanding of everything!