My chap is 4yrs and still very dappled but will lighten with time; and it is unusual to get steel grey or iron grey horses at 8-10yrs as they have normally faded out by then but beyond that I think it varies massively.
With the heavies, scrooge was iron at 3, white by 5
Jacob was dark dapples at 3 and is now white in front but still has a dapply bum age 5 (it was his birthday last week!)
Tim on the other hand was very dark nearly black grey at 2 1/2 and now a year later is only a little lighter
Theres no really a science to how long they will take to fade, as far as im aware it varies from horses to horse.
My girl is steel grey at 3 years old and hate the thought of her fading out she is such a pretty colour
Hoping she does it very very slowly although already her summer coat is alot lighter than her winter coat was
My grey (on a chestnut base) was rose grey as a yearling, but now, at just turned four she is dappled grey with a steel grey mane and tail, and white socks still.
She isn't getting lighter very quickly, but I anticipate she will gradually go white like her sire.
Timescales vary.
S