For (usually) weekly, 4-8 hour sessions, I do between 3 and 5 hours of prep (I voted 4-6, because sometimes it takes longer).
Of that, about 1 hour goes to statting all the monsters and NPCs (I'm running a true20 game, so it doesn't take any longer than that). Generally, I try to borrow and modify existing stats that either I've made or others have made, and I compile the stats of every foe the PCs will face for the session in a single document, which I print out.
The rest goes towards writing up my notes (which tend to be outline-formatted word files, divided up by area, time, etc., with some 'scripted dialogue' in there, but usually just broad strokes stuff about "NPC X will insult the players and attack if they return after they've killed his henchmen," or "NPC Y will congradulate the players for killing NPC X, and offer to put them up in his guest rooms if they've ever in town and need a place to stay."
This, of course, is not how I go about the process chronologically.
First, I try to think of the 'concept' of the adventure (I generally run 1-session episodic adventures, because I have a highly variable pool of available players from week to week).
Then, if it's something centered on a certain type of enemy the PCs will be facing, I'll stat them up first. If it's more general, I'll write up my notes first, and then determine what sorts of enemies I need to whip up.