In this paper we address the estimation of the duration of the production process of a complex product in a Make-To-Order (MTO) or Engineering-To-Order (ETO) environment, a common situation in the manufacturing sector, specifically when production operations are executed by human operators. The addressed products, due to their complexity and high degree of customization, are specifically designed for a customer, together with their production process. On the other hand, the uncertainty affecting the production problem must be considered due to the presence of operations executed by human operators, the high level of customization entailing the need to cope with new products and processes and the behavior of the production environment. The proposed approach deals with the estimation of the duration of a production process modeled through a project network whose activities have an uncertain duration. It can provide support to a manager in the estimation of the makespan of the production of a product to provide a realistic and competitive due date to the customer, as well as to manage the scheduling of production and supplying operations and the resources assignment within the plant. The proposed approach is described, validated and compared with existing approaches through an ideal case. Moreover, a real case application is provided, related to the assembly process of a NC machine, to demonstrate its industrial viability. © 2015 The Society of Manufacturing Engineers. Published by Elsevier Ltd. All rights reserved.