You can wait for as long or short as you want, it's your class. Whatever you feel like is a necessary amount of time. I would ask them why they take so long, maybe they are really busy with work or school at this time. I know I have a lot of projects since the semester is wrapping up but if I was involved in a program I would at least check messages everyday and respond to them. Have you tried leaving a message on their wall? Sometimes I get an email from getting certain things and not others. Also how new was this person that you took on? If they only did one or not any before they tried to get a mentor they might have just forgot all about this place, it happens.
Whatever you choose it doesn't hurt to keep one student on your list and it also doesn't hurt them if you cancel that class because they can just get another mentor.