I have difficulty concentrating. I know that sounds strange coming from a PhD student, but I really can't read for longer than 2 hours at a time. I would say my max is 4 hours a day. At one point, I was worrying that I'm not doing enough, but to be honest, I really don't think there's any way to measure how much you've progressed until you start writing. No matter how much you read, you're most likely to be focusing upon a particular set of texts when you write, so it can be a case of reading less but reading the right stuff, rather than reading heaps of material.