How long those it take aluminum to decompose?

Answer 1

Instantaneously.

Elemental Aluminium metal reacts immediately with oxygen in the air, to form aluminium oxide. Aluminium oxide is a very stable compound, and so does not allow the underlying aluminum metal any further reaction.

If you have aluminium pots and pans at home, then you are actually touching the aluminium oxide, because the underlying aluminium has already reacted with atmospheric oxygen .

Answer 2

Aluminum corrodes rapidly on its exposed surface, but unlike the rust on iron which catalyzes more corrosion, this corrosion layer is protective and once it is thick enough corrosion stops. After that aluminum can last almost forever, unless the layer is damaged.