There's the late period followed by Macedonian Egypt and that space between the two (the last native Pharaoh) is considered to be the end but given how Ptolemy and his descendants adopted the native culture could it be said ancient Egypt actually endured for longer?
The last known hieroglyph is the graffito of Esmet-Akhom, could that be considered the very end when the original native culture was fully eradicated/absorbed first by Christianity and then by Islam?
Not to discourage other answers but /u/Bentresh gave a great answer to Ancient Egypt is often described as the longest continuous human civilization, and seems to have maintained a surprising amount of cultural continuity. How accurate is this description? If so why were they able to maintain continuity so much more than other civilizations? that might be of interest.