I remember back when I played Warhammer 40k on a weekly/daily basis that I always hated it when my Dark Elder or my Space Marines decided that they didn’t really want to fight any more. Having your troops run away and hide is always a pain, especially when it comes to War games.
If you check out the Dungeon Master Guys Podcast, in Episode 2, they give some good general tips for speeding up combat when the general outcome has been decided. They recommend getting inside the mind of your monsters, which I whole-heartily agree with. However I think some situations call for a bit more specific guidelines; although thinking of "outs" as mentioned in the podcast, is a really good idea, having a general idea of when those "outs" are going to kick in is a good thing to plan for.
I think a too many GM’s fail to use the “Run and Hide” option when they are in combat. I know in one of my encounters, a few very pesky Sniper Orcs had themselves a nice little perch and were hailing lead on the party through the entire combat – They also watched as about 90% of their fellows were brutally beaten, fried, and lightning bolted to death before their very eyes, without every stopping their bombardment of gunfire. The players then spent the next 2-3 rounds trying to get up to the two pesky snipers before finally popping them in the skull.
At some point, the combat has been firmly decided and victory is going to be whole sale for the party. At these points, it is really better to just let the party win quickly, so you can move on to a new, fresh encounter, which gets the party thinking, rather than grumbling. I realized after the last encounter, what I should have done is ended the combat when only the 2 snipers were left and had the poor fellows beg for their pathetic lives; of course, the party would most likely get whatever information they could out of them before looking up the Coup De Gras rules in the Rules Compendium, but again, that is a bit of fun that they wouldn’t have had if they spent time climbing a ladder and using an at-will power a couple times. While the Dungeon Master Guys recommend that when the Leader falls, the minions will take to the hills, I also would recommend that when the minions see most of their buddies painting the walls with their insides, those minions are not going to want to stick around, even if their leader is still standing.
I think it was Mark, from RPG Circus, who mentioned that in order to speed up combat with a large group, he pondered whether or not he was going to have to turn a bunch of monsters into minions in order to get the combat done quicker. He ended up not having to, because his party was on task, but actually, his point is really good. There are some situations in the game where monsters are not going to run and hide. These might be the mindless Undead or other creatures that disregard the beating that they are taking. In situations like that, I think it is often a good idea to turn a creature that into a minion, after it has taken a significant amount of damage and has very few ways to continue to deal damage to the party.
Under both of these Circumstances, I think you could easily follow the 75% rule (which I believe is the number used in 40k for Morale, If my memory serves me right). When either 75% of all the important monsters in a combat are dead, or 75% of the last enemies health is gone, either turn the creatures into minions or have them surrender, run, hide, beg for mercy, or some combination of the preceding. Your players will enjoy getting on with the game, and they might even come up with some interesting way to finish things off.