Depends what game I am running.
I am currently GMing Star Wars d20 and a heavily modified AD&D1 campaign, and the prep overheads vary wildly.
For SW I have to read through the adventure a couple of times (when I use a WotC module - most of the time), make notes regarding pertinent plot points and changes to fit the plot into my galaxy far, far away.
All in all, I prepare for 5-6 hours per session.
For AD&D1+, the campaign has been going for 16 years so the players and I have quite a lot of knowledge about the world.
Mostly, I flesh out the major NPCs (the ones that are worthy of a name and background), any monsters (I almost always modify existing monsters or make them up from scratch to keep the party on their toes) and a backstory (what is going on in the world at large). Takes 2-3 hours per session.
Trouble is, the players ALWAYS do something I didn't plan for so I ALWAYS end up GMing on the wing. Then again, because of the investment I have already put into the game, I have a large number of potential plot hooks buzzing around in my head, ready to pull out at a moment's notice.
Obviously, all the above is subject to multi-session games, where the prep for the second session is significantly less.