In order to adapt current subjects to the ECTS credits system, it is necessary to estimate how much time a student has to invest in learning a subject. The Polytechnic Engineering School of the University of the Basque Country has made an initial study to investigate this matter in several subjects. A weekly questionnaire design was used for all the students in the class, supplemented, when necessary, by an individual interview, with student collaboration. The results indicate that the study rhythm and the number of hours are strongly conditioned by the evaluation system. (Contains 3 figures and 4 tables.)