In Spain, the 15M movement organised itself into self-managed assemblies in public squares, going beyond the militant circles to which these practices had hitherto been confined. Based on an ethnographic study conducted over a ten-year period in Madrid, this article examines the effects of this broadening of audiences on the practice of assemblies. By looking at the difficulties involved in resorting to consensus, it focuses on the main political challenge of building a shared culture of collective decision-making. The question of temporality, central to this process, is expressed at the levels of learning a shared culture of assembly and of decision-making itself. We are thus exploring the tension between, on the one hand, the openness of the 15M assemblies to all, and on the other, the production of norms specific to these meeting practices, which require an apprenticeship that makes participation and integration into the assembly difficult.