We observe the nucleation phase of in‐plane ruptures in the laboratory. We show that the nucleation is composed of two distinct phases, a quasi‐static and an acceleration stage, followed by dynamic propagation. We propose an empirical model which describes the rupture length evolution: The quasi‐static phase is described by an exponential growth while the acceleration phase is described by an inverse power law of time. The transition from quasi‐static to accelerating rupture is related to the critical nucleation length, which scales inversely with normal stress in accordance with theoretical predictions, and to a critical surfacic power, which may be an intrinsic property of the interface. Finally, we discuss these results in the frame of previous studies and propose a scaling up to natural earthquake dimensions.