Blue,
This service will be available indefinitely. I like to have two pro bono projects at all times. As for when I will be available to take on another after this one, it is hard to say. It depends on the length of novel, how quickly the pro bono client can revise as well as whether or not they just disappear. My rule is that, if I do not receive a response within two weeks of my last communication, I assume they have given up. At that point, I put out a notice for availability again. That said, typically, each project I take on generally runs about two to three months.
Additionally, you can apply at any time and I am happy to put approved applications on a waiting list. I've done this in the past. Sadly, many of those I've put on a wait list, disappear if they've had to wait a few months.