i find the amount of prep time is also somewhat dependent on genre.
i'm currently running a superheroes campaign with M&M, and that seems to take the least amount of prep of any type of game i've ran. (mainly because there are usually less NPCs to stat out.)
for our current story arc, my prep time went something like this:
from the backstory one player gave me, i knew the PC had a best friend with an abusive ex-boyfriend. currently, the friend was living in Montana (the campaign is set in New Jersey, however).
ok, it'd be a cool hook if i turn the psycho ex-boyfriend into a supervillain -- give the PCs a reason to intervene and a chance to leave the campaign city for the wilds of the open plain.
so, what kind of supervillain idea can i come up with for Montana. for some reason, the first thing i thought of when i thought of Montana was a bunch of paleontologists out on the open prairie digging up dinosaur bones. (though i'm pretty sure there's a lot more fossil digs in Wyoming and Utah than Montana... anyways...)
ok, how about some paleontologists dig up some dinosaur fossils and release an ancient evil curse? abusive ex-boyfriend happens to be dating one of the grad students on the dig and he's the one hit by the curse.
voila! instant rampaging berserk monster villain, The Living Fossil!
players go to investigate after alerted to the monster by friends living in the area. the monster is gradually working his way across the state, heading for where the PC's friend is currently living.
spent about 15-20 minutes designing the villain and his dinosaur skeleton construct minions, and i literally have enough material for at least 3 sessions. all the detail work has been made up on the spot -- including the specific details of the curse, the involvement of the local Crow Indians, the past exploits of the PC's grandfather (himself a former superhero) who also lives in Montana, the link between the Fossil monster and one of grandpa's old nemeses, etc...
it's been a pretty cool story arc so far.