And behold, we find in the Midrash Hagadol (Exodus 7:25) a distinction between the Plague of the Firstborn and the rest of the plagues: "And it is filled for seven days - upon them it says, 'And the LORD will strike Egypt, striking and healing' (Isaiah 19:22), that the plagues would harm them, then heal them, in case they would despair. Rabbi Yehuda says, every plague that the Holy One, blessed be He, brought upon the Egyptians lasted for thirty days, with seven days between each plague, except for the darkness which lasted only three days, and the Plague of the Firstborn was instantaneous, so they all lasted for twelve months. And this is as we learned (Mishnah Eduyot Chapter 2, Mishnah 10), the judgment on the Egyptians was twelve months. The plagues began in Iyar and ended in Nisan." It is clear that for all the plagues, there was a duration of time, unlike the Plague of the Firstborn which was instantaneous.