-10h ripoff
10-20h short
20-30h good
30-50h great
+50h gold

Depends a lot on the game in question. If it's something like Uncharted or God of War which are constant streams of action and incredible story moments, then a 10 to 12 hour story would be a perfectly agreeable length. If we move to the other side of the spectrum at RPG's, then anything less than around 20 hours (including side quests, grinding etc) then I'd be disappointed.

So pretty much it depends entirely on the quality of the hours in question. I hardly expect Pokemon to blow my mind each hour like a Naughty Dog game, but I do expect it to pump out several dozen entertaining hours.

For every genre it's different but overall (RPG excluded):

Too short: Less than 6 hours
Short: 6-10 hours
Regular: 12-15 hours
Long: 15+ hours

If you take RPG in account:
Too short: less then 15 hours (except for some games which are only $ 5,-)
Short: 15 - 30 hours
Regular: 30 - 50 hours
Long: 50+ hours

Short: less than 10 hours
Refular: 10-20 hours
Long: +20 hours

5 to 8 hours short
9 to 13 regular
13 to 1000000 long
1000000 to 9000000 a little bit longer

Under 10 hours, too short
10-15, short
15-30, regular
30+, long

Short 5-8 hours

Regular 8-12 hours 

Average 12-24 hours 

Long 24+ hours

It depends on the game. A 20 hour FPS would be a grueling experience, but a sub 20 hour RPG I would consider short.

For RPG's/Action Adventure:

<5 Hours - WTF Did I Buy?
5-15 Hours - Short
15-25 Hours - A Bit Short
25-45 Hours - Normal
45 -65 Hours - Long
65 Hours Plus - Excessive

For other genres:

<5 Hours - WTF Did I Buy?
5-6 Hours - Short
6-9 Hours - A Bit Short
9-15 Hours - Normal
15-25 Hours - Long
25 + Hours - Excessive

Imagine playing a 25 hour Call of Duty game. You would just get bored by the end of it.

It really depends on the genre, first person shooters often get away with >8 hour campaigns, but RPG's are almost always over 20 hours or so.

Most Mega Man Titles: 3 hours (first play-through)
Super Mario Bros.: 2 - 4 hours
Donkey Kong Country: 4 hours
Most Street Fighter Titles: 15 Minutes
Super Mario Land: 30 Minutes

Still awesome games. Every one of them. The time it takes to play a game to the end doesn't really matter if it has high replay value. I'm playing some of those titles until today and spent more than 1,000 hours with each one of them. A massive RPG can eat up 100+ hours, but I'm never coming back to that.
Besides, if a bad game is long, it makes it even worse. A long good game is great of course, but sometimes you just want something short. I don't have that much time for gaming, sometimes I'm not playing for two weeks or so. I would have completely forgotten about my progress in a massive RPG in that time, so I'm not motivated to play any further. I would have to start over again and again and again. It's the main reason I haven't bought Persona 4 or Ni No Kuni, although they look quite interesting to me. With a full-time job, wife and son, there's simply not enough time for those games.

It's therefore pretty cool that there is such a broad variety of games. That way everybody will find something he likes. There just aren't those Über-games that appeal to everybody. That does also count for playtime. Some like it short, some like it longer. Everything's ok, there are enough titles for every taste.