So someone wanted to answer for Parshas Bo an answer that I liked.
It’s brought down b’sheim R’S.Z. Aurbach, that he asks a stirah. In the morning we say first ?? ??????? ?????….i ?????? ??????and in the evening we say ???? ?????? ?? ????? ????? ?? ??? ????? so which came first? So it’s brought down in mesechta (?) that the Mitzriyim were actually struck at midnight but they remained like a goysis (half-dead) till after the yidden left the land. So we can answer the stira, that they takeh were struck with a makka, but they didn’t die immediately, only after the yidden were redeemed and left.
This answers our original question that the bechor animals could have died anytime between chatzos and when the yidden left Mitzrayim.
Agav I was thinking maybe that could be one reason why it’s called Makkas bechoros and not necessarily Harigas Bechoros. Because for many hours the Bechorim were struck with a Makka that suspended them into this state, of hanging between life and death.