I think it may be somewhere around a 2-to-1 ratio: 8 hours of prep (spread out over a couple of weeks) to get ready for a 4-hour game. A lot of my "prep" though is just thinking -- mulling things over, etc. Another big chunk of my prep is getting stats ready (coming up with a nice menu of monster stats, developing NPC stats, etc.). The rest is kind of putting it together, developing timelines (what happened when, especially important for any kind of mystery or investigative adventure), finding or making a map, etc.