I have a "minimal wear" p90 "Sand Spray"(or something like that) and I have a good amount of "field tested" skins. Over time in my inventory, does item quality degrade(while unequipped) or do the items have to be actually used to get beat up?
No the skin exterior quality does not degrade. It will always stay the same.
Note also that a weapon will not degrade over time; a weapon skin will stay in that state forever.
from CS:GO Wikia