It varies quite a bit depending on the given game we’re playing and also where we are at in play.
For example, for my recent Mothership campaign, most of my prep consisted of familiarizing myself with the module A Pound of Flesh. It details a space station that was to serve as a home base of sorts for the PCs. Aside from that, I came up with some ideas for other locations nearby and some other setting details. Then we created characters as a group, and I then incorporated the ideas the players came up with for their PCs, or those implied by their PCs’ backstories and relationships. Once we started playing, my prep essentially consisted of thinking about what might happen next, and jotting down some bullet points… so maybe five to ten minutes of anything beyond daydreaming.
For my recent Blades in the Dark campaign, the setting already exists and the crew creation is a group effort… and most of the relevant details for play come up during that step. So I did barely any prep at all for that game. In between scores, I’d do the upkeep for the downtime phase of play… so I’d adjust faction statuses based on what happened in play and I’d come up with entanglements based on rolls. So maybe ten minutes of work between sessions.
For the current game I’m running, Teeth:  False Kingdom, I do literally no prep. Everything is generated during play. If anything, I may think about ideas, locations, or characters that may be interesting or what not, but that’s about it. No actual preparation, nothing written down, no notes or bullet points, no maps or NPCs… nothing. 
For my next game, I’m planning on running DIE. Because that game is so focused on the specific PCs that are created for play, I cannot do any real prep until we sit down and create the PCs as a group. Once we do that, which involves asking a crazy amount of questions about the characters and their relationships and aspirations and fears and all of that, then I use that material to prep a fantasy world that will put all of that grist to the forefront. I expect I’ll do a couple of hours of prep after that first session to prepare for session 2 and the campaign overall. After that, I expect I’ll shift back to a similar pattern as I have with other games… giving the game some thought in between sessions, then spending maybe ten minutes jotting down some possibilities for the coming session.