As a child/teen:
I started violin at 7yo.
Nothing to grade 1 (d) in a year, but most who started with me took two years.
Following year: grade 4 (m) - totally skipped 2 and 3.
Following year, maybe a bit longer, grade 5.
About 18 months each for the higher grades, I think, but was playing second study (piano) by then.
Grade 8 (m) at 14, so averaged a bit under a year per grade.
However most of the people who started with me (I'm a school music service kid) only made it to grade 5 by that point.
Piano, started at 11, grade 8 (d) at 17, so six years. I took them all except grade 2.
Flute - taught myself to about grade 6 in a year when I was in the sixth form.
Viola - guess that's cheating - played for coursework in sixth form so assume I could play it back then.
Did loads of orchestral playing, string quartets, Baroque ensemble, multiple choirs (I got a choral scholarship at uni), keys for big band, sneaky jazz playing and fiddle stuff. That was more important than the exams!
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A lot of my students are young when they start and so we make slower progress. I currently have a year 2 and a year 3 in their second term of violin working for copper medals and then probably prep test, a year 4 taking grade 1 violin in her fourth term and aiming for a good mark, and a teenaged beginner taking grade 1 piano in her second term - she works her socks off. However, the children who start at 4yo will take a good three years to reach grade 1, I think. We're still learning our CDEs. There is also a need to allow for psychological readiness - e.g. my year 2 probably won't be ready to play in a formal exam situation for a while, so we'll likely do copper and bronze medals, maybe silver. She has a good tone and plays wonderfully in tune, but she's six years old.