When a story you love ends, it's a tragedy. You've been through the wringer with those characters. After the danger and excitement of the climax, you're looking for a satisfying wrap up to the action something that lives up to the execution of the promise the story premise made in the very beginning. Something that cements the lessons the main character learned throughout the slings and arrows of the story.
But hey, no pressure, right?
Endings are hard. There's your hero after the climax, bruised, bloodied, often broken hearted. That may be figurative. It may be literal or metaphorical, but on some level, your MC has suffered a death at the climax. That is the point in the story wherein it becomes crystal clear that this character is no longer who he or she was when the story started. And there's no going back. The MC has to return to the ordinary world after the adventure and they have to do that bearing some useful gift. The cure for whatever plague was decimating the town. The head of the dragon eating all the sheep. A Holy Grail of some kind - even if that grail is simply a relationship worth hanging onto - the gift conveyed there is the formation of the new family, whatever form that family takes.
This is what we, as readers, want to see in a resolution, then - the MC applying what he or she has learned. If you're writing genre, that's a happy thing. The gift is a benefit to the community. If you're writing literary, the gift affects no one by the MC and may carry a bit of a curse with it. Think Cassandra in the Trojan War. Gifted with the ability to see the future, cursed so that no one would believe her. If you're writing tragedy, the denouement is the fall out associated with the MC failing to learn his or her lessons through the course of the story.
I suspect that most of our readers have been trained by TV and movies to expect short denouement. But since books aren't constrained by producers watching the dollar signs with every frame of film captured, maybe it really is incumbent upon authors to luxuriate in our endings a little bit - to give readers time and space to transition out of the world of the adventure and back into their normal world. Certainly different stories will bear different treatments. If you spend very little time in your hero's everyday world at the beginning of your story, you can probably invest little time in the resolution. If you spend pages on the heroine's normal world at the beginning of your story, immersing readers in the details, you will need a denouement of similar detail at the end to contrast the differences between the two. Set up and resolution are the frame on the action of your story. You don't want a lopsided frame, right?
So really. How long should your story resolution go? Exactly as long as it needs to be to bring your characters full circle. Unless you're a terrible human being and you mean to strand your characters mid-circle. Then your denouement needs to be even weightier in order to measure the consequences of characters who failed to complete their journey(s) or who died in mid-arc. (We're looking at you on that one, GRRM.)
Do I make it sound facile? It isn't. Just go have a look at how well I follow my own advice. O_o