עד כה means UNTIL NOW. A Midrashic explanation (taking this sentence to mean, “thou wilt not hearken until (עד) thou hearest the word “כה”) is: until thou hearest from me the announcement of the slaughter of the first-born, which I will begin with the words, (Exodus 11:4) “Thus (כה) saith the Lord, About midnight etc.”

AND, BEHOLD, HITHERTO THOU HAST NOT HEARKENED. Because this was one of the [ten] plagues and henceforth He will begin to smite him, He therefore said to him that it was his wickedness which was responsible for the bringing of the punishment upon him since he hearkened not to the command of his Creator. Now at this time, [i.e., when the warning about the first plague was given to him], Pharaoh did not declare to Moses and Aaron that he will neither hearken to G-d’s words nor let the people go. It was only at the first time [when they came before him] that he said, I know not the Eternal, and moreover I will not let Israel go. At present, he did not rebuke them; he only heard their words and remained silent, for since they performed the wonder of the serpent before him and Aaron’s rod swallowed up their rods, he was already afraid of the plagues, except that during the first plagues he attempted that the magicians do likewise, that is, by means of deeds of sorcery. Thus he was afraid, and yet he hardened his heart. This is the sense of the expression, And Pharaoh’s heart was hardened.