We report the observation of a characteristic incubation time in the growth of silicon nanowires using the vapor-liquid-solid growth mechanism. This incubation time manifests itself during the growth process as a characteristic time delay in the range of several seconds to minutes, prior to which no nanowires are formed. The observation is in excellent agreement with a theoretical model based on the diffusion of silicon through the catalyst, which predicts the presence of an incubation time, as determined by diffusion of the growth constituent through the solid catalyst. Furthermore the theoretical dependence of the incubation time on the activation energy is derived, and validated experimentally for the first time by measuring the incubation times of silicon nanowires obtained by chemical vapor deposition for both gold and copper as a catalyst. The experimentally observed incubation times are in excellent agreement with the theoretically predicted incubation times. The reported incubation times are a universal feature of vapor-liquid-solid growth and can be applied to any other metal/ semiconductor system for the synthesis of nanowires and provide a novel route to determine the phase space for nanowiresynthesis.