How long does it take for glass to decompose?

Glass can't rot. It is made of silicon and oxygen.

Glass doesnt decompose. It does take a long time to become a solid, but glass now will be glass in millions of years. Some types of glass can be made to decompose by adding chemicals similar to what etching does to glass. The term devitrication refers to the decompositition of glass but this is under forced chemical changes and not something that's going to happen in a natural state. Glass takes thousands of years to become a solid. Glass darkens as it ages as well. Lava glass or obsidian is glass found in nature and can be dated back to the beginning of the Earth, and is still glass.

Glass never decomposes. It is made from molten sand.