Any combat should be able to be over in 1 round. If you surprise+assassin-target+coup de grace the enemy leader with a blowdart while balancing on his tent, he should DIE, despite the fact that he is 1 CR above you. I freaking hate it when you sneak up on the giant spider, climb it's web with Slipper of Spiderclimb, and then coup-de-grace it in the face only to have it knock you out with a poison-bite, wrap you up and eat you. If the party brings the situation up to their maximum advantage, they should be able to dish out enough damage to outright slay anyone of their CR.
That being said, when the opposition has the high ground, is set for a charge, and has Defense Mode "don't get hit" on, with Buff Mode "we are ready for your spells, tricksy wizard" it should be a different story. Probably more on the realm of 6-7 rounds.
Average combat should last around 5 rounds, 1 round of "holy crap, giant spiders!", 1 round of "use your arrows boys!"/"to arms, men!", 1 round of "haha, the suckers are blinded!", 1 round of "they aren't dead yet, Jimmy just took a nasty bite that will take weeks to heal naturally", and 1 round of cleanup.
Boss fights should be around 10 rounds. 10 rounds of "Party Vs. Balor" or 3 rounds of "Down the Necro!" and 7 rounds of "his army is useless without his leadership". Note, at higher levels, the "Party vs. Balor" fight could easily be "Down the Balor!" and "his demon army won't stand a chance now!".
# How many Skeletons or Imps on the board is enough/too many?
This is extremely level-dependent. 100 1st level fighters is not enough to a level 12 party. That being said, the plot "army of skeletons" that approaches the maximum to be controlled feels right when it is about 10-30 3rd level opponents to a 5th level party. Insta-summons that take 1 round should cap out at 4 (a possible 20 at the end of a normal-length combat). 5 summons every round is too many, in my experience (even if they are the killable grunt guys, it's just annoying).
# How many Hit Points is enough to be satisfying?
Don't care. To the player, functionally, if it only have 20 hp at level 5 but has crazy-dodging or high AC, it can be just as satisfying as the giant that keeps taking a sword to the face. That being said, HP should be enough to actually last 4-5 rounds without dying (which is dependent on how much defense the monster has).
# How many Hit Points is a pain in the ass to calculate?
I gave up calculating HP as a DM (as a player it is never a pain
![Wink ;)](./images/smilies/wink2.gif)
Other things:
Puzzle-monsters are stupid and have the feeling that:
Lycanthrope: "Those adventurers have silver! Run away, your DR is useless here!"
I agree with the suggestion that puzzle-monster are normal monsters that keep coming back until you can puzzle-kill them, at which point they are normal monsters.