There’s no set length for an ending, no measure in words, fractions or percentages. Endings are as long as endings need to be but perhaps we should measure it from the point where the action begins to wind down. A more technical way to describe it might be ‘falling action’ as opposed to the beginning’s ‘rising action’.
To make it sound even simpler if we think of it in terms if we imagine the story as an adventure the ending would be the part after the goal has been achieved where everyone finishes up their business and separates. For example, the ending of The Lord of the Rings begins when Sam and Frodo are rescued from Mordor and all the subplots begin to resolve, such as Aragon and Arwen’s wedding, the Elves going to the undying lands and the Hobbits returning home.
In this instance we may argue that Tolkien’s ending appears to have a false start because after the action begins to wind down the Hobbits return home to discover that The Shire has been sacked and the other Hobbits need rescuing. However, this extra adventure is in itself the resolution of a subplot as opposed to the beginning of new adventure. This extra action answers the question of what happened to Saurman and Wormtongue. Without this extra action the reader would be left wondering what happened to these two important antagonists because they weren’t simply going to run away licking their wounds and learn their lesson. This gives the chance for closure on this plot and the villains getting their comeuppance for their villainy which is often an important element in storytelling.
While we could argue that no-one would’ve minded if two antagonists had not been seen to get their comeuppance but we must also remember the sort of story Tolkien was telling. Not only was LOTR based on the old tales of Europe where evil is always defeated but it was also heavily influenced by Tolkien’s Christianity where, theoretically, goodness is rewarded and evil punished.
So, despite this apparent false start the Sacking of the Shire is in fact a winding down of events and resolution of subplots. How many subplots you resolves depends on the sort of ending you’re writing, as we’ve previously discussed, and it is these that dictate the length of your ending. If you’re uncertain how much needs resolving and how much time to spend on it a first draft is the perfect time to find out. A technique to discover this is to simply write the ending as long as it takes to tie up all the plots you want to and then in editing you can cut out what you feel you don’t need or doesn’t work.
It is important to remember that in first drafts you don’t have to worry about overwriting, just write it and it can be tidied up later.