Article:
LARP Plot Tips: Timing

One of the easiest ways to scale the difficulty of a module is to implement and adjust a time limit. With these time limits, the players will need to accomplish their goals quickly and effectively. The nice thing about modules designed this way is that they can be run for any group, simply by increasing or decreasing the time they have to complete their goals.

Here are some ways to effectively add timers to your modules.

Time Before the Next Wave
In a battle where you have enough NPCs to stagger attacks, you can use a timer to determine the time between fresh waves of enemies. Groups that work well can dispatch the first wave before the second appears, but if they do not fight quickly they might find themselves being overwhelmed. When scaling these modules, you can either change the time between waves, or the strength of any given wave. Just try to keep from having three different waves of NPCs, since your NPCs will either be bored sitting out too long, or the players will be dead from fighting three waves at once.

Time Between Setting Changes
Changing the setting based on a timer can be something as simple as rafters falling in a burning building, or as complex as shifting to another plane. The point of this technique is to make the players re-assess the situation when the timer ends. Make sure to actually change the open pathways. If your setting change doesn't force groups to repostion or change their strategy, then this ends up as unwieldy flavor rather than a fun and interesting effect.

Time Before Catastrophic Event
This is probably the most used mechanic on modules that are timed. Something bad is going to happen, whether it's complete destruction of the setting, formal magic rituals being completed, or hostages being killed. Essentially, when this timer runs out, the module is over. While these are good for rushing players, the problem with these timers is that they're hard to scale on the fly. With wave timers and setting timers, you can slow things down or speed them up as necessary to make the module challenging. Catastrophic events only happen once, so you won't know if the timer is too fast until it's too late.