How long does it take for steel to decompose?

Technically steel or any other metal does not decompose. If you are referring to corrosion, it will eventually turn into Iron Oxide (rust) over time, but that is largely dependent on atmospheric conditions. The more humidity, the quicker. Less oxygen, slower. In the ocean with the salt water, VERY fast. The Titanic at the bottom of ocean cannot be raised due to this. The Titanic is no longer made of steel, but almost entirely of pure rust.

The company I work for built half inch thick horses formed from corten (a high quality grade of steel). These figures are expected to still be standing over Vantage, Washington (on I-90) 50,000 years from now.

On the other hand, a thin sheet of regular (non stainless) steel agitated in vinegar can entirely rust away within a couple of days.