How long does an average encounter take?

[Lateral]

In real time, not rounds. I'm writing a dungeon crawl for a campaign I'm going to be running later in the year, and I'm trying to set up the encounters so that they make it through about a floor a session, but I'm not actually sure how long it'll take them to run through each encounter.

[Curmudgeon]

This is going to vary greatly. It's mostly a linear increase with character level, because both PCs and their enemies gain more actions per turn. More attacks, more spells, more AoOs. At near-Epic levels you can expect hours per encounter. At 1st level an encounter, even with creatures making full use of cover and concealment, can still be over in 10 minutes.

[BowStreetRunner]

This is going to vary greatly. It's mostly a linear increase with character level, because both PCs and their enemies gain more actions per turn. More attacks, more spells, more AoOs. At near-Epic levels you can expect hours per encounter. At 1st level an encounter, even with creatures making full use of cover and concealment, can still be over in 10 minutes.
And this isn't even factoring in party size. I've played in games with 4 players and 1 DM that moved fairly steadily, while games with 8 players and 1 DM seemed to take forever to go through the same number of rounds.

[Jack_Simth]

In real time, not rounds. I'm writing a dungeon crawl for a campaign I'm going to be running later in the year, and I'm trying to set up the encounters so that they make it through about a floor a session, but I'm not actually sure how long it'll take them to run through each encounter.

Depends greatly on how much prep work everyone does.

For instance:
Suppose a Barbarian-12 has one d20 and one d12. He can roll his full attack with his greataxe, no problem... it's probably going to be six throws, however, and will take a while.

Suppose this same Barbarian 12 has a cup, a red d20, a blue d20, and a green d20; a red d12, a blue d12, and a green d12. He can roll all six at once, and resolve all three attacks in one toss, assuming he doesn't threaten a crit (and if he does, it's just one more toss regardless of how many crit threats he got, same system). This is much faster. He may also want some color-coded d10's in case of miss chance (while theoretically you need two... it's very rare to find a miss chance that's not an even multiple of ten, so the ones digit is almost never meaningful).

That's a simple example. If everyone does some prep work well (pre-calculates effects of common buffs that are used, plans what they'll do while the other people are taking their turns, has stat cards for any common summons, keeps color-coded dice for full attacks, et cetera), then a turn goes very quickly (a few minutes for the entire table), and as combats usually only last a turn or two (although this depends significantly on the optimization level of the party... but people who put in this sort of prep work generally optimize fairly well), it'll be over quickly.

If nobody does prep work, everyone has a minimal set of dice (or just not color-coded dice), and everyone waits to decide on what to do until their turn comes up... it can easily take half an hour or more for a single round. As the sort to not do prep work are also a little less likely to optimize, the combat is also likely to last more turns, too.

So the answer is "it's swingy, extremely so". To the point where an average from anyone who hasn't played in your group is not going to be very useful.

[boristhemaniac]

Another problem that has come up in games I've played is focus. Oftentimes the players can be busy trying to roleplay their characters to the point where their turns take twice as long as everyone else's or distracted by something out of game, which can also cause them to miss important details about the encounter. I don't know if this is true for your group but even at low levels such things in my group can make encounters take 30 minutes.

Another thing to consider is how elaborate the encounters are. In one instance I played, we fought ghosts and wraiths in a cave which had ledges about 20 feet off the ground where the enemies flew up to for ranged attack. This fight took almost an hour to complete, and half the characters weren't able to help once the ghosts flew up.

[Psyren]

This is probably something you'll have to measure for your specific group and iterate on. It depends on too many factors for an average provided by the internet to have much meaning.

[Flickerdart]

From my experience, the average combat round takes about the same amount of time regardless of level - most people rarely plumb the depths of their tactical options, instead preferring to rely on a small handful of the best things. A 20th level wizard might have two dozen spells of levels 1-5 available, but he's not going to actually care about any of them and will likely focus on the one or two nines he has prepared that will make the biggest impact.

The number of rounds (and thus real time) in a combat varies depending on the encounter's difficulty compared to PC game mastery. I would say that more than two combat encounters per session is going to be a tall order whichever way you slice it - if you have 4 hours between "everyone is done with their day and can gather at the game master's lair" and "the fighter is asleep on the couch, the wizard's mother is here to pick him up, and the rogue has to leave before the trains stop running" then it doesn't matter whether an encounter takes two hours or an hour and a half because you're still only getting in two of them.

I would stick to 2 encounters and pad out the bits in between. If the PCs finished encounter 1 quickly, add some corridors and a trap or two. If a few unlucky rolls kept the monsters alive longer, then cut those corridors (and use them next week).