This depends largely on you, your style of learning, and your other time commitments. Some students like to do an accelerated mentorship over the summer, finishing their projects before school starts up again. For students that are working, interning, volunteering, or traveling in the summer, this might not be ideal.
Mentorships completed during the academic year happen at a more leisurely pace, in order to accommodate for school, extracurricular activities, and testing schedules.