Maybe you could try giving them some sort of deadline a couple weeks out and just let them know that as long as they are your student you are unable to accept anyone new, so therefore....

And on that note, if they are taking that long finishing a review, that does kinda make me wonder how good they would be if they did get advanced and accept free assignments. I try to have mine done in a couple to a few weeks tops, and to me at least, a couple months for one review is completely unacceptable.

I send people who request me as a mentor a pre-written introduction to the program, the rules, my expectations as well as both of our responsibilities. I ask them to contact me once they have accepted an assignment so that I can keep track of the due dates; that they begin working on their assignments by week 3, so that we have time to send it back and forth for corrections if need be, and that they maintain contact with me at least once a week just to let me know that they are still around and prepared to do the work. I also let them know that if there are circumstances in their lives that will make it impossible or too difficult for them to focus on their reviews for a while that they need to contact me so that I am aware, and if need be, can help them contact admin over it so they do not run into problems.

If I have not heard from a student for 2 to 3 weeks, I will contact them with a request for an update, if they do not reply within a few days I will send them a warning that if they do not contact me back within one week, I will cancel their class. The only students who I am more relaxed about are the ones who are already advanced reviewers. This is because in general they require less work, are actually prepared to do the work, are usually quite familiar with the whole system, and above all, because I am not putting my reputation on the line by requesting products for them, which they just take off with and never review.