I've been thinking about this one a bit looking at how things have morphed over the years. Long ago most games really weren't too terribly long, they just took time because they required a higher more precise level of skill in a flat environment more often than not. You look at stuff now, it costs more for sure, but you find aside from a few select genres stuff seems relatively short.
Are you someone who weighs it upon the hours you get out of it? Is it not about time but all the added junk that's put in there to keep going back for or hunting some corner or sequence to get it? Maybe you're just good as long as the ride is fun whether short of long if you get enough hours out of it to be happy and if that means finishing the game all the better?
Personally I don't know 100%, but once upon a time pre-game industry I was all about finishing the game, not in totality unless it was something fairly straight forward like a Mario game or FF2 SNES which had minimal side crap to derail the experience. That caused me to have some level of permanent burn out over the last 13 years since that exit that unless something really compels me I find it hard to focus enough to finish a game, even far less to really dig into the extra fluff. I find for me if I can make a certain milestone I set in my mind I'm fairly happy, not totally pleased but it works and if it's good enough I can go back. Yet when I do finish something like the first 3DS Adventure Time game last night before bed I was thrilled I took it down as it has been a long time since I knocked a game out like that. So what does it for you to make a game a good purchase?