Depends on replayability. A game that is great and short but compels you to replay it multiple times is worth the money and I would not begrudge such a game being 2-4 hours long. A game with modest or "forced" re-playability (e.g. locked out difficulty settings requiring at least 2 plays to get all difficulty trophies), is fine coming in at 7-10 hours. A game that you are likely to only want to play through once should be >12 hrs. Obviously most developers will claim that their game is replay-worthy but by and large unless a game only needs to be played through more than once for platinum trophy hounds but for people not fussed about trophies there is no reason at all to go back to it then the replayability claim is very hollow.

This is a general rule of thumb though. If a game is mindblowingly awesome and comes in at 5-6 hours with little or no replayability then it can be worth paying retail price for it. Though a mindblowingly awesome game has intrinsic replay value just so you can experience the game a second, third, fourth or even more times.

In some cases long game length can be a detriment, especially with games that have some essential replayability built into them. Thinking of RPGs where you can play as different characters and you can make different choices that affect how the game progresses. If a game is too long then it becomes rather too much of a chore and too much of a time sink to get through all the permutations that you want to get through. With Mass Effect I would ideally like to do at least 3 plays through. One male paragon, one female paragon, and one renegade and possibly a 4th time to have both sexes run through as renegade. If the game is too long, then it thwarts my intentions because I can't justify the time.

For RPGs, 40 hours or more worth of gameplay/content. Any other genre, 15 or more hours of gameplay. That's fairly easy to accomplish if a game has a multiplayer mode though. I add both the single player and multiplayer together.

If you have a busy life, that's besides the point. That has nothing to do with the value of a game and it's no excuse for a game to be short. You can always save your game and play it some more when you have time for it. 

To me, if a game is not worth the 60 dollar price tag for it new, I might wait to get it used when the price drops to a point I'm more comfortable with. 

As most people have stated it depends on the game. If the singleplayer is short, then I would look for other ways to continue to play the singleplayer that still added value. Playing co-op with friends or if every time you played the SP offered a different experience. If a game has, SP, MP and other content I enjoy to do then the price can easily match the time I will spend with that game. I personally look at the entire product to determine if the game is worth my money not just one piece.

Length means nothing to me. All about fun factor and replay value.
If a $60 AAA is long and boring, it's not worth the $60 price tag.

But a game with tons of text to read and repetitive combat can over stay its welcome for most consumers. This is why I stopped playing JRPGs.

However, I might spend 100 hours with Halo 5 multiplayer because I'm actually having fun.

Frankly, I don't have a ton of free time. Sometimes I just want games to throw all their best content at me in about 5-8 hours. The longer titles in my collection tend to collect dust. I'm looking at you Witcher 3 and Dragon Age.

Fair enough. 

Eh, depends really. Though I generally feel like putting 25 - 60 hours into a well crafted single-player game is about right.

For me if The Last of us were 5 hours i am fine with it, (even the reality is 15 hours), that's just the analogy. It doesn't matter how long the time is if the can packed the content of the games enjoyable (Memorable Story, Gameplay, content, bonuses, Graphic, replay value, High Quality Production, Innovation in terms of story, gameplay, or other things) i am fine with 5 hours.

I know. Was really enjoying it but something happened ( it's been years so I can't remember what) and I didn't play it for a while. I just never went back. I keep saying I will one day, though.

Story driven games can be anything between 8 and 16 hours, but I expect to enjoy every moment of it.
Sandbox games should be longer than that, especially RPGs which should be focused on building a stronger character over time.