Your single player is largely what ties you to the art of your craft. I'm going to say that campaigns themselves can be longer than you may like, but then I also feel like with the way the media is and the ability to look up impressions and specifically how long a game is; you realize what you're getting yourself into. Let's say that I play Assassin's Creed 3 and along comes black flag. While I'm not sure of the total time it takes to complete the campaign, I am sure that there's going to be assassination contracts, side missions, possible hidden quests, courier missions and etc.
When you look at GTA as a franchise, each game is typically larger than the other. The entire way the game tells its story has changed in fact. The game started off as this ridiculous turf war kind of experience and then came GTA 3. After that, each story became a little more compelling and sometimes rode the coat tails of the worlds before it. Finally, the latest GTA just shows you this world that keeps its quirkiness and sense of invisibility while portraying these characters in ways that are devilishly real.
I'll give an example of another game that has a "long" campaign by 10 hour standards. Tomb Raider's reboot in 2013. That game played a lot like a Zelda or Metroid game. You went through a level and as you got more access to weapons and items you were able to do more things around the previous maps. They had a lot of collection quests and to positively impact you, you got access to some really freaking awesome skills. This game (if you played straight through) could have been under 10 hours, but then you find yourself thinking... I really want to stab a guy in the neck when I dodge them so I'm going to complete this quest. Then you're running along and you see the markings for one of the hidden tombs and you're like "I really want that extra 500 salvage to upgrade my gun." So you go do it.
I spent more time then necessary in that game because it compelled me to do so with the rewards that it offered. You spend 40 hours in Assassin's Creed instead of the 7 it takes to play the primary missions through straight, because it compels you to do so through the positive reinforcement it gives its players. Whether its money, weapons, collection of items or whatever it might be.
I'll leave with this. I believe that because of twitter and the various cons where developers can meet the people who play their games and communicate with them, there's an open line of communication. A campaign is going to be as long as the people who love that game. When you play Skyrim you know exactly what you're getting into and the developers know exactly what their core audience demands. So, I believe campaigns are the perfect amount of time. Unfortunately, that's not appealing to everyone, but to the core they are shooting for, those people love them for what they are. There's no shame in spending a lot of time on a short campaign or quitting something half way through because you lose interest. WHen you find something that compels you to move through its content and its bonus content, that game was meant for you.
