generally, very little at all. an hour per session would be a lot for me.
of course, i'm only talking about specific session prep. once i've got the setting prepped, i don't really need to do much work for each individual session.
before i start a new campaign, i throw together a bunch of generic NPC stats. so when i'm getting ready to run an adventure i can just grab some 3x5 index cards with "Guard, Human, 3rd level Warrior" and "Bandit Leader, 6th level Rogue", etc. if i need new stats specifically for this session, i can usually whip them up pretty quick.
Monster/NPC Design: like i said, i usually do at least 75% of this before the campaign officially begins. it's usually only "named" NPCs that i have to worry about later.
Story/Dialogue: almost entirely ad-libbed on the spot. i rarely plan dialog ahead of time. my "story" prep rarely exceeds three of four sentences for a typical session. usually takes just a few minutes of jotting down notes.
Encounters: i can usually figure out how many / what kind of encounters i need quickly from my story synopsis and grab the NPCs i need.
Maps/Supporting Art: i rarely draw maps. if i need to, i'll visualize and draw something on the spot. i do sometimes spend time on hand-outs, but not for every session.
PC Experience and Awards: when i hand out XP, it's either entirely by the book (and thus doesn't require any additional thought beforehand), or i come up with some yardstick like "they'll gain a level every three sessions" and throw out the normal method of determining XP. either way, it doesn't require any time before the session begins to figure out.
that's how i do it -- i wouldn't recommend it for every DM. (heck, it's backfired on me a few times too -- i had a Star Wars d20 campaign that i ad-libbed into a corner once. if the half the group hadn't moved away right then, i wasn't sure how i was going to continue...)
it
can be a kinda cool feeling, though, to have the players think they're in the midst of some involved, detailed, intricate conspiracy even though you're making it all up moment to moment and have no more idea of where it's all going to head than they do.

when it works, it's awesome, but when it doesn't it can really suck.
