From the Twelfth Dynasty onwards the word appears in a wish formula 'Great House, may it live, prosper and be in health', but only with reference to the buildings of the court rather than the king himself.
10So Moses and Aaron went to Pharaoh and did just as the LORD commanded. Aaron threw his staff down in front of Pharaoh and his officials, and it became a snake. — Exodus 7:10
However, the earliest certain instance where pr-`3 is used specifically to address the king is in a letter to Amenhotep IV (Akhenaten) in the mid-Eighteenth Dynasty (1539-1292 BC) which is addressed to 'Pharaoh, given life, prosperity and health, the Master'.