How does an apostate return to Islam? And is there a specific period for that?

If a person apostatizes from Islam and then decides to return to it, they must bear witness that there is no god but Allah and that Muhammad is His servant and messenger. If their apostasy was due to denying something known to be an indispensable part of the religion, then their return is not complete except by affirming what they denied. There is no specific duration for returning, and their repentance is accepted as long as they have not reached the rattling of death (ghargara). If they return to Islam and perform what they can of its rituals, then they are a Muslim.