It all actually depends on the speed of your work and how much you progress. I wouldn't recommend rushing in 4 months as taking time to learn and really understand every part of the two books is essential. But if you are sure that you are a fast learner, then go ahead! Maybe instead of thinking about how many hours, but how many chapters you can complete... Maybe 2 or 3 a day? It can speed things up. But i would say about 2 hours should be spent on that. And practicing pronunciation with the CD and stuff.. and finding someone to talk to regularly can help