Several years ago a prosecutor actually argued in court that "intent can be formed in an instant."
I remember that incident. Iirc the defendant claimed self-defense. He pulled a knife during a beating swung it wildly/blindly. Ended up fatally stabbing one of the assailants. Prosecutor claimed he decided to kill in an instant, rather than just get away, while receiving a beating. Don’t recall the outcome.
As to the issue of how much planning is required for 1st degree murder, I guess it depends on the wording of the statute. If all it requires is “intent” to kill, that can be formed in an instant. It’s just that it’s a lot harder to prove instantly formed intent than showing evidence of e.g. lying in wait for the victim, or other clear planning over hours/months.