Some GMs will have an entire campaign planned out before the first session. Others won't even know what is going on in this session until shortly before. Most of us fall somewhere in between.
Which kind are you?
I tend to plan ahead a little bit. I like to have the next few sessions sketched out, particularly if I am doing a contained arc. If the PCs are going to a specific location, I'll sketch out the story of what is going on there which gives me guide posts for each session, and then I will flesh out the likely to be hit items before the session. Sometimes I over plan and end up with two or three sessions, but that is hardly a problem.
Just recently in one game I gave the PCs - in character - a teaser for what the entire end of the game is going to be. Once that fully kicks off - soon! - I'm not sure how long it'll take for them to get through it. I just know what pieces are on the board, and how those pieces will be moving.
In the other game, I have things planned to get them out of the current area. Beyond that I don't know. It will depend on what the PCs do.
How about you?