There is no specific time to prepare current affairs but yes you must be well-versed with the last 3 month's current happenings around you. Here are some of the prepation tips for the preparation of Current Affairs given below you can prefer:

Monthly Magazine & Newspapers/e-papers- You can read any number of publications you want to. For the exam read The monthly magzines & news papers like Hindu, HT, The Economic Times, Indian Express etc. 

Current Affairs - Focus on National news, Editorial, Business and Sports sections. You should be updated with all the current information of it.

Make notes - You will be going through news from more than one source (magazine, paper, websites), so naturally, it becomes difficult to go back to those exact sources for revision. So put everything in one place. You can arrange your notes month-wise.

Good Luck!