Because I have a group that acts unpredictably, I stay away from laying out the whole adventure – let alone the campaign – beforehand. Instead I write each new session as a response to their actions in the previous one.
This is a problem when I can't predict the speed they will progress through events. Sometimes a session I expected to take five hours only takes two because I was out of material (I'm surprisingly bad at coming up with things on the fly), while other times it takes three sessions to get through what I thought was one session worth of material. Combat and freeform decision-making scenes are especially unpredictable.
How can I estimate how fast a group of players will go through a section of adventure, to facilitate a more appropriate amount of preparation? I've run into this problem in systems as diverse as World of Darkness, D&D, and Shadowrun.
Best Answer
Wing It
This may not be the most technical answer, but I have exactly the same problem as you do. My group can be so erratic when it comes to doing what I think they're going to do as well as the time it takes.
I elected to do what you've already mentioned, and write in between sessions, basing what you think off of their last decisions. It helps to keep notes of everything they did, everything they picked up on (NPC's, totally irrelevant things, etc), so you have a better idea of what they're going to pick up on first. I also tend to find that once my PC's have a goal in mind, they rarely deviate until that quest has been completed as such.
In terms of combat, you can always come up with a few encounters to sort of plug-and-play when the time comes. Maybe make a couple of easy, medium and hard encounters for the sort of area the PC's are in and then you've already got something to fall back on if they decide to go in a different direction than what you had planned. (thanks to Sardathrion)
So really, I think your best bet is to wing it. Plan as much as you can without completely railroading them, keep consulting your notes, and perhaps just try and get a feel for what takes them longer - combat or exploration or RP and then try and pace the session based on that. At the end of the day, players will be players - they'll have you flying by the seat of your pants for some sessions and then others will go exactly how you planned, that's just the nature of the game. Good luck!