I have been wondering what type of adjective long is, quantity or quality?
It is a qualitative adjective:
Qualitative adjectives describe the qualities of a person or thing – whether they are large or small, happy or sad, etc. This type of adjective is gradable.
a fairly tall man
a very boring film
a really long holiday
an extremely expensive car
A quantitative adjective modifies a noun by indicating a number/quantity It can be either cardinal or ordinal number.