It depends. Even when I just "wing it", like for my L5R game, I still spend an hour, sometime 3 or 4 hours, to just figure out how things/NPC's are reacting to what the players are doing, or did, between game sessions.
In 3.5, I had to spend hours just prepping to make sure I "knew" the rules for all the special maneuvers and/or powers the creatures/NPC's contained. I also always had to pre gen all the treasures, to make sure they were used by the creatures/NPC's against the party, and to let the players know what they are.
Even in C&C I spend a half hour to an hour doing some prep work, making decisions, etc...
When I need to print out and laminate some Fiery Dragon counters, or Fat Dragon Games, Skeleton Key tiles I want, that can add about an hour as well.