Smell

haven't you ever heard of entropy

Well, i don't know about there not being any more scents but taking your question to the extreme, if there is proton decay, then all matter that we know of today will eventually decompose into subatomic particles and if there isn't, all matter will become iron which will become black holes which will then become subatomic particles. In short, with our current understanding of physics, yes everything will decay into basic subatomic particles.

I think theres some speculation on how we smell. Some think its like molecules fitting into puzzle piece receptors, others think theres some sort of quantum property that our sense of smell uses.

G-protein coupled receptors

How long would it take a pencil at room temperature, assume normal indoor environment, to turn to dust?

Smelly stuff eventually stops smelling once all the chemicals that can go into the air are gone.

Most things.
Some things keep evaporating until nothing is left. Naphthalene (mothballs) or Camphor.

Gold's not unique. Things that are already oxidized are permanent if left alone. Most rocks are like that. Easy to find rocks that are, not just millennia, but millions of years old, provided they've been sheltered from erosion.
Aluminum quickly "rusts" but it's only a surface layer and the process stops there because aluminum oxide is less dense than pure aluminum. Thereafter, aluminum will sit on your shelf for millennia. (Iron crumbles because iron oxide is denser than pure iron, so the surface layer contracts, cracks and lets in oxygen and water so the process continues until its all rust.)

if you cover yourself in gold you will live forever.

About
3.50 billion

Pretty expensive though