The first thing you have to decide when designing your adventure is how long you intend to play it in real life.
You play an adventure over one or more sessions, each of which lasts at least 3 hours. A single adventure taking place over the course of a single session is called a one-shot; a single adventure that takes place over a few sessions is called a short; and a series of interconnected adventures taking place over an indeterminate number of sessions is called a campaign.
Types of Adventures
A one-shot is an adventure that's embarked upon and completed in a single, three-hour session. It's the hardest length adventure to design, because three hours is not a lot of time to work with. One-shots tend to be exclusively linear in nature and consist of very few scenes. It often makes sense to run a highly abridged version of session zero during the one-shot itself, skipping things like conflicts, flaws, and story tags.
A short is a single adventure played over the course of three to five sessions. Shorts can be designed as one or more linear or nonlinear adventures, each comprising a handful of scenes, and may necessitate kicking everything off with a formal session zero (or running it as an abridged downtime during play). The difference between a short and a campaign is that the GM puts strict boundaries on the scope of the adventure—things may take place in a particular timeframe or physical space, or the GM may establish a clear goal at the outset that completes the adventure (free the slaves, escape the tower, resolve the mystery, recover the artifact, etc).
A campaign is a sustained, evolving, and open-ended series of interconnected adventures that takes place over an undetermined number of sessions. Campaigns can involve many adventures made up of many scenes, both linear and nonlinear. Campaigns tend to come to a natural conclusion when all the PCs have resolved their conflicts. Overworld play can help shepherd the evolution of the campaign outside players' individual PCs, and players can level up their PCs all the way to level 10 if adventuring time permits.
On Session Length
You can play for longer than three hours per session, but this guide assumes that's your benchmark. We’re also assuming that your table is made up of four or five players, which is the sweet spot when it comes to dividing the spotlight across the table. If you plan to have more players than this, you're going to need longer sessions (or more sessions) to scale up the spotlight.
On Leveling Up
In one-shots, PCs don't level up because there isn't any time for them to evolve. We tend to start and end PCs at level 1, though it's entirely viable to start them at a higher level so as to engage them in more challenging encounters.
On One-Shots & Shorts
One-shots and shorts are variations on the same theme: in both cases, there's a particular narrative experience to be had, it's just that a short gives us a little more time to explore that experience than a one-shot. You can grant heroes in a short a level once per session (per the session-based advancement approach) if you like, but don't feel obliged to if it doesn't make sense in the context of the adventure.
Campaigns, on the other hand, are all about resolving conflicts through story hooks. Players expect to make progress and improve their PCs over time. Be judicious about how you hand out levels in a campaign because you never know how long it will last. At the time of this writing, the original A Quest of Queens campaign is 32 sessions long and we're about halfway through the campaign. (We have run it once a month over the course of several years.) The PCs in that campaign are only level 5, using the milestone-based advancement approach.
Every table will have its own cadence when it comes to completing adventures. Some tables love downtime and will take their time when it comes to running those scenes; others have a more pragmatic mindset and want to push through encounters with efficiency. You will develop a sense for how long it takes to resolve conflicts through story hooks as the adventures unfold. Just remember: it will always take longer than you estimate. Give yourself room to breathe.