Since I'm a bit of a duality on the matter, I figured I'd pop in and comment.
If I'm prepping anything at all, it can only mean one of two things: 1) that I am running from published adventure content, or 2) that this campaign is a "sequel" of a campaign that I've already run.
In the first case, I keep a ratio of 15 minutes prep to 4 hours play, and I do that by not changing anything significant in the adventure (so I just skip any adventure that wouldn't be fun without significant changes).
In the second case, I write a preface (maybe 1 hour, once for the entire campaign) that covers the general idea of the campaign and anything the players need to know in order to make a character or be ready to return to playing the original characters for another time, and an outline. The outline provides a very bare-bones skeleton of the events going on in the campaign, and usually takes about 2 hours to write while making sure I'm not putting anything in the outline which relies upon events before that point going a specific way (i.e. I do not write anything about an NPC's involvement in the campaign beyond the point that the party might be able to kill or incarcerate that NPC, so that I won't have to alter those details later once that part has played out).
Before each session, I'll look at the outline to see if there is anything in particular that will be going on during that session that would affect whatever the characters are doing at that point, but I spend no more than a minute or two on that - and I run each session by setting up the scene and reacting to what the players have their characters do (which is the way any non-sequel non-published campaign I run goes: I pitch a premise, the players make characters that fit to that premise, I set up a scene, and then they drive the story from then on until the story reaches a satisfactory conclusion - with me doing zero prep).
I think, in total and on average, that these sequel campaigns end up being about 5 or 10 minutes prep per session, though the bulk of that is all done before session 1.