How
      does an apostate come back to Islam?
Question

                If a muslim converts to
      another religion and later decides to come back to Islam, what is the
      process that he/she needs to go through inorder to return to Islam???? Is
      there a certain time period within which he/she can return to Islam?
Praise be to Allah.If a person leaves Islam then decides to come back to 
          it, what he has to do is to bear witness that there is no god except 
          Allaah and that Muhammad is His slave and Messenger. If his apostasy 
          was because he denied one of the basic principles of the religion, then 
          his return to Islam cannot be complete until he affirms the principle 
          that he had denied. There is no particular time period within which 
          he can return to Islam; his repentance will be accepted until the point 
          when the death-rattle sounds in his throat and his soul departs. If 
          he is guided back to Islam within the time when it is possible (i.e., 
          before death) and he performs as many of the Islamic duties as he can, 
          then he is a Muslim.