How much prep do you need for an RPG session?

Prep time is the wrong unit

Ask ten GMs how long they prep and you will get ten different, equally defensible answers, because the honest answer depends on the system, the campaign's complexity, how well the GM already knows the setting, and how much the players tend to go off-script. A number borrowed from someone else's table tells you nothing about your own.

What actually predicts a good session is not time spent but what that time produced: material that is likely to matter once the dice start rolling. An hour spent writing prose nobody will read produces less than ten minutes spent naming three NPCs' wants. Measure the output, not the clock.

A compact coverage checklist

Work through these before a session. Most take a sentence or two; none require a script.

• The immediate situation Where the party is, what's already in motion, and what changes if they do nothing for an hour.
• The important people and factions Who's in this scene, what each one wants right now, and what they'll do if the party surprises them. Wants, not biographies.
• A couple of likely locations Not every place the party could conceivably go — the two or three they're actually likely to head towards next.
• One or two complications An obstacle, a ticking clock, or a wrinkle that stops the straightforward plan from being free.
• Clues or leads Something that points somewhere, so a stuck table has a thread to pull rather than a blank page.
• What happens if they do nothing The world doesn't pause for the party. Knowing this stops you from stalling when they duck the hook.
• Blank space Deliberately unprepared room for whatever the players actually decide — the thing no amount of prep can substitute for.

That's the whole list. A session with all seven covered in a page of bullet points is better prepared than one with forty pages of backstory and none of them.

Three kinds of prep, and only one is tonight's job

Conflating these is where most over-prep and most guilt about under-prep both come from.

• Essential prep The checklist above, specific to tonight. This is the only prep a session actually requires, and it's usually the smallest pile.
• Reusable campaign prep Factions, locations, NPCs and the relationships between them — material that outlives one session and pays for itself repeatedly. Worth investing in, but it's background infrastructure, not tonight's checklist.
• Optional worldbuilding Maps, timelines, invented languages, the political history of a country nobody's visited yet. A legitimate hobby in its own right — genuinely enjoyable, often the reason people GM at all — but it is not session prep, and treating it as a prerequisite for running a session is how prep turns into procrastination.

It's fine, even good, to spend an evening on the third category. The trap is believing a session can't run until that pile feels finished. It never will, and it doesn't need to.