“20” means there’s always an encounter, no need to roll.
Let’s say they are on a dungeon level where there is a 1/8 chance of monster every hour.
Paste the 1/8 table on your dungeon master screen or in your notes. Or visit this webpage.
Then if they wait for five hours, look at the “5” line. “14a, 5, 7a, 5a, 3a”
“14a” means that they roll a d20 with advantage (two d20s take the highest). On a 14 or less, there is an encounter.
If there were, and they survived, then they need to make a new check, a “5”. No “a” or “d” means just roll a single d20. On a 5 or less, there is an encounter. If they survive that, keep rolling, with a “7a”.
As soon as there is no encounter, you stop rolling. That’s the point of this. Instead of always making five checks when they’ve waited five hours, maybe it’s just going to be one or two. But you never know what’s going to happen.
The “time unit” doesn’t have to be “hour”. It’s the same tables regardless of it’s minute, 10m turns, hours, days, shifts, or weeks. You know if they’ve waited that many “time units”. That time scale granularity is also what determines to what extent they get to rest between. For example, if you’re playing an edition of the game that has mechanics for short rests and the time scale is based around thrice-daily “shifts”, then yeah they’re going to get to short rest in between. If it’s a busy dungeon level with a time scale based around 10 minutes, then no.
What if they wait longer than 12 hours? As an example, let’s say they wait 16 hours. First do a “12” line. When it’s over (probably because one of the rolls came up empty, but maaaybe because they did survive twelve things), you then start a “4” line. When combining lines in this way, please don’t use lines that have a “20” guaranteed encounter, but other than that, start with the longest lines possible.
…than just actually rolling the amount of checks?
If you are rolling behind the screen (or under a cup or something that you can later lift, or you can use cards that you can later show), yes, definitely just roll that many normal dice normally without using these tables.
This is for when you are letting the players roll the encounter checks which can be a fun way to play D&D.
Even so, it might sometimes be cumbersome to try to find the right table etc. But the good thing is that this is interchangeable with the regular method. Let’s say they go an an 8-hour trek and you need to make 8 encounter checks. You can use this table for a very tense way of doing it (because one very good thing about this method is that they don’t know how many encounters are coming next). Then if you later accidentally misplace this table and they go on a 2-hour trip back to the same swamp, you can then roll two normal checks.
It’s just an extra option to use for when you want the tension & mood of handling long waits this way. Good luck♥︎
I’ll try to port this code to javascript or something in the future but as of now, here it is in Chicken Scheme.