Ideally 1:1 ratio of prep to play time, though my hectic life often requires me to cut corners.
Detailing interesting and mechanically complete NPC stats are probably one of the most time intensive tasks, though it is often the first one I cut corners on. I can fake a spellcaster or psion with a spell list. Faking an interesting and engaging scenario is more difficult.
Generally, at a minimum, I try to make sure I contemplate the events that occur during the game, the challenges, etc. I rip off various modules and other resources to fill the gaps.