Most existing formal theories of group choice focus on predicting the outcome of decisionmaking. However, the duration of decision-making is also of substantial interest, as it indicates the ability of a collective actor to respond to changes in the social, political, and economic environment in an efficient and timely manner. I use an agent-based model of coalition building to generate hypotheses about the duration of multilateral international negotiations. The model produces predictions about the effects of changes in the number of negotiators, the initial preference distribution, and the general level of impatience of negotiators. According to the model, increases in the number of actors and decreases in impatience lead to increases in the duration of decision-making. In these instances, the model provides a clear mechanism for generally accepted cause-effect relationships. However, it also provides somewhat counter-intuitive predictions: preference heterogeneity affects decision-making duration only marginally and preference polarization not at all. In this respect, the model provides novel alternative hypothesis that can be tested against existing theoretical accounts.