The use of refactoring as a way to continuously improve the design and quality of software and prevent its aging is mostly limited to Agile Methodologies and to a lower amount to software reengineering. In these communities refactoring is supposed to improve in the long-term the structure of existing code in order to make it easier to modify and maintain. To sustain such claims and analyze the impact of refactoring on maintenance we need to know how much refactoring developers do. In few cases such information is directly available for example from CVS log messages. In this study we propose a model on how to mine software repositories in order to obtain information of refactoring effort throughout the evolution of a software system. Moreover, we have developed a prototype that implements our model and validate our approach with two small case studies.