Current affairs paper has more questions of issues relating to pakistan then international . And answer to your question is that prepare all important issues from janruary of 2012 to december of 2012 because next year's issues will not be regarded as important ones because papers are finalized before that .
And some times past issues are asked in indirect way as their consequences on present scenario but their % is low and automatically you covers them while studying current problems , as historical perspective is quite important to understand the issue
And believe me if you found in paper that you studied more then required , then you also have to consider the importance of that data because you can use any of the relating data in essay ,pak affairs, islamiyat and even in optionals