We have worked our way through six plagues. The only indication of time frame that we have is that seven days passed after God struck the Nile. I have read that some scholars think the time from the Nile turning to blood to the deaths of the first born was 40 days. God is kind of partial to 40 days, so who knows. At any rate, time waits for no man. Pharaoh was just a man, and he was running out of time. One of these days, Pharaoh will let the Israelites go, he just doesn’t realize it yet.