My table has a habit of "let's go option E" and surprising me, so I my prep is divided into a higher level "here's what they reasonable could do" (which at times ahsn't been wide enough, and "here's the most likely few options of what I expect them to do".
The high level planning is idle time between sessions, occasionally jotting down a short note unless inspiration strikes and I need to record a good amount.
The weekend before the session (well, sometimes continuing during lunch the day of, we play Monday nights) I take those notes and open up my documents.
I have a This Session document (separate from my setting and long term documents). Front page is just a recap about all of the characters, including factions (pos/neg/conflicted) and features that I often scan when I want to make sure I've got spotlight time for all of my PCs in what I'm planning. We can often go one or more sessions without combat, so having times to spotlight other parts of their character is important.
It also has short summaries of current arcs for reference, plot threads I've left hanging from previous sessions, and some quick cheat sheets like a line for names by race and gender. I also have recurring NPCs (non-combat) for the area, copied out of my larger full NPC document.
It has short recap of last session section, which for a while I was reading at the beginning of each session, but I admit I got a bit lazy about it.
Then I spend an hour or two fleshing out the most likely thigns to do. I make sure to include a "color" section, be it those they see on the streets of a city or the different types of oppressive darkness in the underworld.
If I don't already have them, I'll flesh out anything I expect to turn into a combat in terms of hazards, goals, and foe stats. I was most recently running 13th Age (a d20 game), not 5e, and it has legal PDFs. So that's often cut-n-paste, sometimes with reskinning or with tweaks for what I want. Really quick though.
All in all, prep is several idle hours during the week thinking ahead and thinking large scale (in the shower, driving, etc.), and 1-2 hours before the session to write down and flesh out. If we played weekends instead of weeknights it might have to be longer to match a longer session.