Just the ending verses of this chapter are enough to illustrate how quickly a false confession or statement of repentance can disappear.
Exo 9:27 And Pharaoh sent, and called for Moses and Aaron, and said unto them, I have sinned this time: the LORD is righteous, and I and my people are wicked.
Exo 9:28 Intreat the LORD (for it is enough) that there be no more mighty thunderings and hail; and I will let you go, and ye shall stay no longer.
Exo 9:29 And Moses said unto him, As soon as I am gone out of the city, I will spread abroad my hands unto the LORD; and the thunder shall cease, neither shall there be any more hail; that thou mayest know how that the earth is the LORD’S.
Exo 9:30 But as for thee and thy servants, I know that ye will not yet fear the LORD God.
Exo 9:31 And the flax and the barley was smitten: for the barley was in the ear, and the flax was bolled.
Exo 9:32 But the wheat and the rie were not smitten: for they were not grown up.
Exo 9:33 And Moses went out of the city from Pharaoh, and spread abroad his hands unto the LORD: and the thunders and hail ceased, and the rain was not poured upon the earth.
Exo 9:34 And when Pharaoh saw that the rain and the hail and the thunders were ceased, he sinned yet more, and hardened his heart, he and his servants.
Exo 9:35 And the heart of Pharaoh was hardened, neither would he let the children of Israel go; as the LORD had spoken by Moses.
and said unto them, I have sinned this time; not but that he had sinned before, and must be conscious of it, particularly in breaking his promise so often;
but now he acknowledged his sin, which he had never done before:
and this confession of sin did not arise from a true sense of it, from hatred of it, and sorrow for it as committed against God; but from the fright he was in, the horror of his mind, the dread of the present plague being continued; and the terror of death that seized him, the rebounding noise of the thunder in his ears, the flashes of lightning in his face, and the hailstones beating upon the top of his house, and against the windows and sides of it, frightened him exceedingly, and forced this confession from him:
the Lord is righteous, and I and my people are wicked; which was well spoken, had it been serious and from his heart; for God is righteous in his nature, and in all his works, and in all those judgments he had inflicted upon him; and he and his people were wicked in using the Israelites in such a cruel manner, and in detaining them when it had been promised them again and again that they should have leave to go, and especially in rebelling against God, and disobeying his commands.