It is important to have a correct approach to practice. You will benefit more from several short practices (e.g., 15-30 minutes per day) than one or two long sessions per week. This is especially so in the early stages, because of the basic nature of the material being studied.
In a practice session you should divide your time evenly between the study of new material and the revision of past work. It is a common mistake for semi-advanced students to practice only the pieces they can already play well. Although this is more enjoyable, it is not a very satisfactory method of practice. You should also try to correct mistakes and experiment with new ideas.