Marketing has studied the permanence of a client within an enterprise because it is a key element in the study of the value (economic) of the client (CLV). research that they have developed is based in deterministic or random models, which allowed estimating the permanence of the client, and the CLV. However, when it is not possible to apply these schemes for not having the panel data that this model requires, the period of time of a client with the enterprise is uncertain data. We consider that the value of the current work is to have an alternative way to estimate the period of time with subjective information proper of the theory of uncertainty. To achieve the above we propose, through a numerical example, a methodology that uses in its first phase the Fuzzy-Delphi method, with a group of experts, to accomplish an estimation of the lifetime period of the client within an enterprise, expressed as a trapezoidal fuzzy number, and to get a better consensus of the previous result, as a second phase, we submit such information to another group, different from the previous ones, obtaining a reduction of the size of the interval defined by the initial trapezoidal fuzzy number.Keywords: customer value, customer permanence, delphi method, trapezoidal fuzzy numbers, uncertainty, expertsJEL Classification: M20, M31(ProQuest: ... denotes formulae omitted.)1. INTRODUCTIONIn the literature, the permanence or active life of a customer within an enterprise has been studied under different angles. For example, Grayson and Ambler (1999), based on previous studies of Moorman, Zaltman and Deshpande (1992), argued that within the marketing of services the relationships between clients and enterprises, at the long term have a dark side, which can provoke a reduction on the level of confidence in such relations.On the other hand, Moorman, Zaltman and Deshpande (1992) explain their results mentioning that there are dynamic factors that exist in long term relationships. For example: they argue that in lasting relationships each one of the parts acquires experience, which can provoke a loss of objectivity; besides, they justified that very long relationships can transform the perception of each one of the contraries as very similar between them, this can decrease the add value of the relation; another argument that they mention is that if the relationship of service between client and enterprise is sustained for a long term, this can lead to a mayor expectation on the service that they provide jeopardizing a bigger possibility of dissatisfaction; the authors mention that the clients can believe that the service provided in a long term relation, can take advantage in the confidence of one of the parts and act in an opportunistic way. As we can see, these arguments are the opposite to the main principles of the relational marketing.In opposition to what is said in the previous paragraph, the study of the confidence in the relational marketing led to the theory named The commitment-trust theory, developed by Morgan and Hunt (1994), taking this element as key on the success in long term relations. It's important to understand that the links established between clients and enterprises, will have different qualitative characteristics between those who are long term relations rather than the ones of short time, adjusting the best dynamic to each case.Another point of view in the study of the permanence of the clients is the one who has to do with the benefits that they cause. Such line was developed by Reichheld and Sasser (1990), Reichheld (1993), (1994), (2002), and much more followers. They explain that the permanence of the client, generates four additional benefits to the profit basis, these are: revenue for sales increase due to customer spending often of cross and up-selling, for price premium extracted for value-added services, for operative costs reduction by improving the efficiency of serving customers, and revenues for referrals. …