When does the Tasleem in prayer begin and end? And is it permissible to turn one's head before uttering "As-salamu alaykum wa rahmatullah"?

Turning the head right and left during the taslim (salutation of peace at the end of prayer) is a Sunnah. The prescribed manner is for the worshiper to begin the taslim facing the qiblah (direction of prayer), then turn, and complete the taslim as the turn concludes. If someone deviates from this description, their prayer is still valid, but they miss out on what is recommended and preferred. The jurists have agreed that if a worshiper performs both taslims to their right, or to their left, or straight ahead without turning, their prayer is valid, but they miss out on the virtue. It is not appropriate to turn first and then utter the taslim; this is because such a turn is out of place and is makruh (disliked) in prayer.