The length also depends heavily on the price, If i pay 50 euro for a game i'd expect quite many hours of entertainment, at 5-10 euro i won't expect the game to last as long. (There are however plenty of cheap games with insane replay value that offers hundreds of hours of entertainment at almost no cost at all)

I think it depends on the type of game. If it's mission-based like say Gundam: Side Story (DC), then it should more than 9 missions. That game was incredibly given the story, weaponry, and customizations. But if it's a platformer, it shouldn't take me a whole month straight to beat it either. Even with doing the extras.

Well, for a casual game, most player will spend 15 minutes per day to relax. A game for 1 month will be 7 hours. This is the ratio you need for casual game. remember to keep the words per minute below 100 as players read an average of 400 words per minute, they still need the other 3/4 of the time to play the game. Simplify the plotline if there are any, or don't have any plotline if you can make a game that does not need any.

Notes
Beginner ~4 hours per month of game
Casual ~7 hours per month of game
Advance ~11 hours per month of game
Softcore ~ 13 hours per month of game
Hardcore ~15 hours per month of game
Professional >15 hours per month of game

Select your audience, the length of the game reflect the attention span of your game player. Most average player will leave after 1 to 3 months of gaming the same game, unless it's in the Beginner or Causal groups.

11.01010375 inches.

11.01010375 inches? Why does that sound like the length of a PC moniter... (I wonder...). It is going to be like Zelda with only bow and sword. Except that it will have more moves, and combos (Prince of Persia with bows maybe?). I have a good idea though of the length it should be. Thanks.

I remember when I was a "bit" younger that I wanted some games (story/campaign mode) to go on forever, and wished the developers would continue popping out expansions indefinitely. One such game was Heroes of Might & Magic (and 2 and 3, subsequently). Fortunately, 3DO did keep pumping out a great amount of sequels, expansions and mission packs regularly, and I readily devoured each one of them until the fourth sequel killed the whole series with "innovation".

The easy answer is to go on as long as the game still provides entertainment. Getting more of the good stuff can't possibly be bad. It really comes down to the core gameplay's replay value, because that's what the player is going to be repeating over and over, regardless of the theme, story, background or customization.

Another thing to look out for is not being creative enough when your game mechanics allow you to be. For instance, I've just been playing some Dragon Age, and got mildly irritated by getting the exact same challenge after opening three different doors in the same dungeon. I've fought roughly the same number of enemies of the same type (and appearance) that behaved the same, and - you're probably guessing - I defeated all of them the same way. Which is again similar to mostly all generic encounters the game provided so far.

I don't want to be doing that for another 30 hours. I don't even want to be doing that for another 30 minutes. Which is why I'm not doing it right now and I'm writing a forum post about it instead.