How quickly are electrons emitted after suitable light reaches a metal surface?

When suitable light reaches a metal surface, there is virtually no time lag between the arrival of the radiation and the emission of electrons. Experimental observations have established that this delay is immeasurably small.12

This instantaneous response is a key feature of the photoelectric effect, which cannot be explained by classical wave theory. Instead, the phenomenon is understood through Albert Einstein's corpuscular theory, which posits that light is absorbed in discrete quanta of energy called photons. Because each photon transfers its energy to an electron in a single, direct interaction, the emission occurs immediately upon the absorption of sufficient energy to overcome the material's work function.12