4th edition Dungeon & Dragons is relatively well structured, so you can predict how long it takes to level up. A full campaign has 30 levels, it takes about 10 encounters per level, which means the overall length of a campaign is 300 encounters. But with my group playing only every other week for an evening, and doing about 3 encounters per evening, that results in the campaign being 100 sessions long, or about 4 years, if we ever get to the end. Very few dungeon masters would ever bother to plan a campaign 4 years ahead. For my campaign I have an idea for a theme that could last until the end, but I certainly don't have all the adventures planned out.
Now there is an old DM saying that the most important adventure is the one you are playing right now. That is where most of the preparation effort should go into. But planning several adventures ahead in lesser detail has its advantages too: You can introduce story hooks and hints about future adventures early on, build up master villains that last for more than one adventure, and create a generally more cohesive world.
Now some people write all of their campaigns and adventures from scratch. I don't. I might have sufficient writing skills for a blog, but J.R.R. Tolkien I ain't. I rather use premade adventures, which I then modify. That way I can take advantage of all the handouts, maps, and tokens provided with the adventures, and rely on somebody having play-tested the combat encounters. Even if the story in the WotC adventures is sometimes a bit weak, I don't mind. The story is the part I'm modifying the most anyway, to knit the adventures together into a campaign.
In this "knitting" part I had a major breakthrough this weekend. I had collected a bunch of official 4E adventures, some based on recommendations from my readers, plus some "best of" modules I played with a different group back in the 80's. At first I had some problems fitting them together, with the level requirements not fitting, or the setting of the adventures being so different that it appeared hard to string them together. But then I realized that my first adventure had two somewhat disparate parts, which I might better split and insert a different adventure in between. And doing that the end of the second part could be modified to give the perfect starting point for one adventure I had trouble including otherwise. Which then would fix the level gap to the next adventure after that. Suddenly the whole thing made perfect sense, and now I have adventures lined up leading from level 1 to level 8, or about 1 year of campaign. Complete with an overarching story, and lots of story hooks linking the adventures between each other.
Of course that is just the rough outline. I will still need to prepare a lot of the details, especially those of the old adventures which I need to convert to 4th edition. But I'm happy that I now have an idea to where my adventures are leading, and I have something more than a patchwork quilt of adventures. This is starting to look good!