Depend on what kind of game I’m running. For my casual guys, I usually use the time I have to get to where we’re playing to think of something. For my long-running campaigns, I do more prep before, especially if we’re starting a new arc. I like to think about the basic premise and scenarios one or two weeks before the game (we play every two or three months on the weekend) while I’m walking, so that takes more or less an hour (unless I get distracted by dogs, bikes, trees or other things). Then I percolate on that a bit and usually plan to write things down two or three days before the game (but often I just type some things up during the train ride shortly before the game).
I have yet to run something that needs a lot of prep for the Gauntlet. I guess I might prep more for that, though.