Its very difficult to tell how much time you need exactly.
Where do you think you are weak ? CR is more to do with logic and RC with your reading ability.
If you can read and understand English well, then you should not find much difficulties in CR and RC and you can master them using any of the prep books that suit you.
If you think that you are weak in English Grammar, then yes here you can improve greatly in a short period of time. You need to learn grammar and good that you are going to start right from school grammar. Keep the learning approach and you should be fine in sometime.
If you can tell more specifically about your trouble areas then there are many people here who will pitch in with some wonderful suggestions ... Just try out.