I run three games. 2 of them purely for fun, the other I get paid for.
For my personal games, one has been running since 2000 so it pretty much plans itself at this point. I have a 30+ page Story Bible that outlines all the plots and subplots I have running so I just review ti once ro twice each week. In the event that there is a big battle coming up, I will spend upwards of two hours each week getting ready.
For my other non-profit game I spend about 4-6 hours a week preparing. I have the storyline and main ideas written, but alot of the encounters and stuff haven't been drafted yet.
For my pay game, I dont get paid for development time, so I manage it a little better. I spend between 2-3 hours each week in preparation for that.
I am currently doing development/design work on two other campaigns. I will not be running either one, so I am assisting the GM's in their design. In our groups, most of the design and prep takes place before the game ever begins. There is one DnD game that I am helping to develop that is taking about 8-10 hours aweek out of my schedule, while the Cthulu game I am assisting on is taking about 3-6 hours a week.