The importance of customer relationship management (CRM) has increased during the last decade. Thus information systems and particularly the quality of customer data are highly relevant for conducting CRM efficiently. Due to the importance of using up-to-date data, this paper analyzes how the data quality dimension timeliness can be quantified. Based on several requirements (e.g. normalization and interpretability) as well as a literature review it proposes a procedure for developing metrics for timeliness that can be for instance adapted to the specific characteristics of the customer data attribute considered. A case study demonstrates the application of our metric by presenting a business case for a CRM campaign of a major mobile services provider.