I ran the first session of my White Plume Mountain (followed by Against the Giants if the players are up for it) campaign for 5E tonight. I decided I wanted to run a higher-level game of 5E, having gotten my fill of the satisfying aspects of low-level play; WPM starts at 8th level.
Anyhow, one of the things that struck me as a little bit problematic when I read the module was that you're supposed to check for random encounters, 1/12 chance, "every 10 minutes." And I was kind of like... huh. There's no way I'm doing that, for the simple reason that there's no great way to judge when exactly 10 minutes of in-game time have passed.
Here's what I told my players: I'm going to use "random encounter" as my default failure consequence for skill rolls that have no other obvious natural consequence. Not checking for one just imposing one flat-out. But I will also check for the 1-in-12 chance whenever you all do something that takes some time, but isn't inherently risky.
As the session went on, I found I was developing a reasonably consistent heuristic about what sorts of things "deserved" a random encounter check: going from one already-explored area of the dungeon to another was the main one. The other, which I feel a little guilty about, was dithering. :-) They got the message though, and kept it in-character: "Let's make a decision before something else nasty comes and hurts us!" Or something like that. So I feel confident I'll be able to know when to have a random encounter check in future sessions.
One thing that helped is that they failed the very first skill roll they made, triggering a random encounter before they were even past the entrance cave! Which they took quite a bit of damage from. So that set the tone very nicely. Quite a few other failed rolls and their consequences have really emphasized just how challenging a dungeon this is! My players are really trying to play smart and preserve spells, too; it's nice to see from a group that does have a whole lot of power available, but knows they're not invincible by any means.
The dungeon is pretty harsh, and there's really nowhere the PCs can "long rest," so they're going to need to retreat at some point. So, I'm also thinking of telling them that, every time they retreat and come back, the random encounter probability will increase: to 1/10, 1/8, etc.
I'm curious how other folks have handled the "check for random encounter every X in-game minutes" direction in games without strict time-keeping rules.