One-shots are typically run when you either a) are trying to fill the time of a normal gaming session or b) aren’t sure that you should put all the effort into creating a vast world. The essence of a one-shot is that it is a self-contained adventure that only lasts one gaming session.
If you know your group well, you should have a basic idea of how long it takes them to complete certain tasks. Plenty of groups spend a lot of time developing what they consider “fool-proof” plans. In order to speed them along, introduce timed events so that they feel that they always need to be moving along. Using hourglasses or timers will help make sure that the time constraints in the game world translate to the real world.
Simplify Your Setting
Putting it all Together
A one-shot adventure should have enough content to cover a single gaming session. Try not to go overboard when planning out a one-shot session. Use NPCs and real life timers to help move your players along. If you need help with creating encounters in general, check out our posts about 30 Minute Dungeons and Creating Encounters. Happy DMing!