From my last several sessions-
Keep on the Shadowfell, session 4, encounter 1- six pcs, levels 2 to 5, vs. 3 goblins, a hobgoblin, and a hobgoblin gladiator (I used the gladiator stats with hobbo racial traits). 5 rounds.
KotSs4, encounter 2- same pcs vs. 9 hobgoblins and a deathjump spider (my own conversion, CR 2). 5 rounds.
KotS, session 5, encounter 1- six pcs, levels 3 to 5, vs. 8 hobgoblins and a hobgoblin priest (I used the priest stat block with a few changes to spell selection). 4 rounds.
KotSs5, encounter 2- same pcs against a single vampire spawn. 4 rounds.
KotSs5, encounter 3- same pcs against 6 zombies and 1 ghoul. 3 rounds.
MINI-ANALYSIS: Since KotS is a 4e adventure, its encounters are well designed and tactically interesting. They also often have waves of bad guys arriving over the course of several rounds. I've noticed that many of these encounters tend to go a round or three longer than those of other adventures. There are basically no non-challenging or trivial encounters.
Return to the Aquan Pyramid, session 4, encounter 1- This is a homebrewed adventure, so don't bother trying to find any details online. A different group of pcs, seven in number, from levels 6 to 9. This encounter was against 6 ghouls and two troll ghasts (I used the stat block for ghasts, but made them large, and had intended to make their Str 18, but forgot to do so in play). 2 rounds.
RttAPs4, encounter 2- same pcs against a custom electric giant octopus ghost (CR 4). 2 rounds.
RttAPs4, encounter 3- same pcs minus one (she was falling asleep- she often drops out early if we play late) against four lightning quasi-elementals (my conversion, CR 5). 2 rounds, because the lightning elementals fled after recognizing the magic staff of the couatl that one of the pcs has.
RttAPs4, encounter 4- same six pcs as encounter 2 vs. two giant poisonous starfish (custom monster, CR 6). 3 rounds, ending with the pcs breaking off combat after the barbarian goliath started scolding them for hurting 'harmless animals', but if it had gone to the death, it wouldn't have lasted more than the rest of that round unless the whole party rolled really poorly.
RttAPs4, encounter 5- same six pcs as encounter 2 vs. 8 helmed horrors. This was a big one that took a while, yet the horrors hardly scratched the group. 7 rounds.
MINI-ANALYSIS: All of these were underwater encounters except the quasi-elemental one. Many pcs were either attacking with disadvantage or using suboptimal weapons, such as an adamantine dagger, to avoid having the D on attacks. Each would have probably lasted one round fewer without that, and the last one- in which the helmed horrors also had the D on attacks- would have been much more challenging in air.
A Deep and Dangerous Road, encounter 1- again, a custom adventure, with pcs escorting a caravan into the Underdark along a very dangerous passage. Six pcs, levels 4 to 8, against a hezrou demon and a drider. The drider spent most of the combat on the ceiling of the cavern they were in, and would come out from behind a stalactite that provided total cover, fire off a flurry of arrows, then retreat behind said stalactite again. 5 rounds.
ADaDR, encounter 2- same pcs vs. two death watch beetles (my conversion, CR 2). Due to extremely poor saves, 3 pcs and a familiar were dropped to 0 on the beetles' first turn, when they used their death rattle ability (which has one of those "if you fail by 4 or more, you instead fall to 0 hps" clauses). 3 rounds even so.
ADaDR, encounter 3- same pcs vs. 2 shriekers and a lithic jelly (a custom ooze that can petrify, CR 4). 2 rounds, no petrifications.
ADaDR, encounter 4- same pcs vs. 3 Abyssal scavengers (my conversion, CR 1), two white ormrs (wormy guys, custom monster, CR 2), one pink ormr (same, spellcaster type, CR 3), and ten yellow ormrs (same, basically minion types, CR 0 IIRC). 3 rounds. Tactical situation favored the pcs- the enemies had to climb a short cliff to reach them.
ADaDR, encounter 5- same pcs vs. one umber hulk. 3 rounds, with the pcs almost all being confused on round 2.
MINI-ANALYSIS: This session had the most typical spread of encounters for my game, ranging from pretty trivial to pretty hard. The pcs also couldn't choose when to rest, due to the caravan's needs; they could get a short rest during their lunch break, and otherwise, nothing until they stopped for the night, when they could get another one or two short rests and a long rest.
EDIT: So that's 52 rounds of combat spread over 13 encounters- an average of four rounds per encounter. But that one 7 round encounter really changes the average. Without it, it's 45 rounds over 12 encounters, or between 3 and 4 rounds per encounter.
