Hard to say. I've never yet played a game that seemed too short.
Obviously excluding ones like Dreamfall where it was a case of wtf it ended just like that!!
I've only played COD 1 and 2 and those were too long already.

I'd love to get an RPG with about 12 hrs length,
with all the fetch&carry running back and forth and wiping out yet another mob of trolls removed.

Actually, lengthwise as RPG's go, Fallout New Vegas and Skyrim were pretty close to perfection.
Meaning the main plot, 10hrs or so if you don't sidestep. I did of course, spent 200hrs at least on both. :)

It depends on the game, really. Usually I'm happy with around 8-10 hours, though more is always welcome (as long it doesn't feel padded or get repetitive). Then again. I'll happily sink up to 100+ hours into a good strategy game like Railroad Tycoon 3, Hearts of Iron 2 or Roller Coaster Tycoon 2. Then of course, there are cheap little indie games, such as To the Moon or Thomas was Alone, and for those I'm perfectly happy with even just one hour, as long as the game is enjoyable in some way.

I suppose there is no universally ideal game length.

^This. I wished TRA lasted 100+ hours it was so good. Some others, keep it at 10 or I will seriously lose interest. It's similar to movies. Lawrence of Arabia works as 3+ hours, the Dark Knight DOES NOT. I loved them both but your subject matter can determine the appropriateness of length.

Depends
RPGs I hate to be under 20 hours minimum (Fable 3 I am looking at you) doing everything
Shooters 8-12 hours is usually best
Other genres it varies a large amount

Depends a lot on the game itself, but my ideal game should be as long as it can be without it starting to feel repetitious or boring. For some games this can be as little as 1½h, and for others well past 100h.

For games that have an inherently high replay value (Civ, Europa Universalis, Majesty and so on), I prefer it when the game is not so lengthy that I feel that I have to put aside a copious amount of time in order to get another playthrough. With some games like these I'm perfectly fine with not actually "finishing" it though, as long as I can get what I want from it (Europa Universalis is a prime example of this, I very rarely play this to the end, but rather I set a goal for myself, and then I try to reach it).


Price does matter though. I would not feel happy about spending even 10€ for a 1½h game.

As I get older, the amount of time I have to play games is becoming less and less. I find that if a game doesn't engage me within the first 1 to 3 hours, it goes on the shelf never to come off again. There are so many games out there (I have roughly 250 on GOG alone) that I want to play, but haven't gotten to yet, that it doesn't bother me. The problem is that I feel a little guilty when I play this awesome game everyone hypes up, only to find out I'm kindof "meh" about it.

i also find that I get interrupted a lot, where I may play for a few hours a day during the week, but then have to go 3-4 days without touching the game. There are a lot of games that, for me anyway, lose their effect if they aren't finished really close to when you started. So the game may have been awesome, but I don't go back to it because it's been so long since I was involved in the story or gameplay.

So for me, the story has to be gripping and the gameplay has to be dynamic or enjoyable enough to want to play the entire length of the game. I find a lot of games get really tedious towards the last 2-3 hours, while I find myself wanting more from others.

TL;DR: It just depends on the game.