The standard flare model, which was proposed based on observations and magnetohydrodynamic theory, can successfully explain many observational features of solar flares. However, this model is just a framework, with many details awaiting to be filled in, including how reconnection is triggered. In this paper, we address an unanswered question: where do flare ribbons stop? With the data analysis of the 2003 May 29 flare event, we tentatively confirmed our conjecture that flare ribbons finally stop at the intersection of separatrices (or quasi-separatrix layer in a general case) with the solar surface. Once verified, such a conjecture can be used to predict the final size and even the lifetime of solar flares.