Your image of what a game master is, is the primary influence on your storytelling style. Although I have done campaigns with elaborately crafted worlds and meticulously planned story arcs, this isn't my usual style.
I see the game master role as more of a director of character-based improvisational storytelling. My job as a game master is to present an interesting scenario for the characters. The players don't have scripts telling them what to do or what to say, so I should not have a script telling me or them what to say either.
This style presents a lot of work initially as the onus of initiating the scenario lies with you, but as the characters develop and their motivations are explored, the work lessens as things become more focused.
The hard part as others have espoused is staying alert and aware. You have to be aware of where they are looking to go and alert and ready for when the characters go where you never anticipated. These unexpected turns are often often some of the most engrossing and rewarding situations in gaming.
As to concretely answer the OP, I will often spend time preparing roughly equivalent to the amount of time we spend playing, with the first session of a campaign spent creating characters together with set premise, tone or theme. Depending on resources and experience this takes about 20-60 minutes per character.
I require more time if I am less familiar with the game system.