For a large molecule (the size of a protein, say), decoherence happens within 10^-19 seconds if it were floating in the air around us—but in a perfect vacuum at the same temperature, it could stay coherent for more than a week.

Why does decoherence occur?

Decoherence happens when different portions of the system's wave function become entangled in different ways with the measuring device. ... As a consequence, the system behaves as a classical statistical ensemble of the different elements rather than as a single coherent quantum superposition of them.

What is decoherence physics?

The term decoherence is used in many fields of (quantum) physics to describe the disappearance or absence of certain superpositions of quantum states. Decoherence is a consequence of the unavoidable interaction of virtually all physical systems with their environment.