There is no strict rule on how long you should wait before cancel students class. It is up to you. But I'd recommend you to contact a student at first to explain and talk over all the nuances - how the process of requesting items and submitting reviews is working, terms of writing a review, how much time you're ready to wait for student's response, ask to inform you if a student has any difficulties that prevent to write a review in time, etc. And when you are sure that your student is pretty serious about reviewing, you can move on and request a product for him/her.