I prefer weekly sessions, and will go with biweekly meetings, but anything more drawn out and my gm performance drops greatly. The sessions tend to be in the 5-8 hours range (which is my preference as well).
With the above in mind, my prep time is usually 3-5 hours per week. I focus on the background and story elements first, and then the game mechanics second. It's all essentially free-form, not being structured, being more of a basic outline really. If the players have provided me information and/or motivations for/from their characters, the process is much easier for me.
Once I have the non-mechanic elements down, I move onto the "playtesting" part where I run the numbers (combat, skill checks, etc.). That's about it.
If I am providing campaign material for the players for each session, the prep time increases by a variable amount.
All of the above methods helps me retain the necessary information so that I rarely have to refer to written materials during the game sessions (which I find extremely beneficial). This leaves more playing time, unless a rules issue shows itself.