In his 2000 craft book On Writing, Stephen King says,
As a reader, I’m a lot more interested in what’s going to happen than what already did. Yes, there are brilliant novels that run counter to this preference, but I like to start at square one, dead even with the writer. I’m an A-to-Z man; serve me the appetizer first and give me dessert if I eat the veggies.
Yes, backstory should play an integral role in your narrative.
Short stories and novels without any kind of backstory will often leave readers unsatisfied. When all you do is focus on the current narrative and never delve into the history of your characters, the reader might feel cheated in a way.
Part of the joy in reading fiction is discovering three-dimensional human beings on the page, and as soon as those human beings begin to read more like just characters, like mere pawns in a plot, the magic for the reader can disintegrate pretty damn fast.
I talked about why backstory is so vital to your storytelling yesterday. You really do need it to enrich your story and characters to the furthest degree possible.
On the other hand, you have to be careful not to spend too much time looking back.
This is where including backstory and especially flashbacks in your writing get so tricky. Have no backstory or flashbacks ever, and the story suffers. Have too much backstory and too many flashbacks, you might rob your readers of the urgency and tension of the current narrative.
I’m with Stephen King on this one. For the most part, I much prefer to look forward in a story than look backward. As soon as the author is stopping to allow the protagonist to reflect on his or her life, I often find myself checking out. Especially if that reflection amounts to pages and pages of flashback mostly told through giant block paragraphs, consider me a goner.
Sure, there’s an occasional instance where I’m so enamored with a character that I will follow him or her through any flashback the author comes up with. Such was definite the case with Theo in Donna Tarrt’s The Goldfinch. And such was the case with all the main characters in Celeste Ng’s Little Fires Everywhere, still the best novel I’ve read in 2019.
Sometimes a novel just hooks you in and never lets go, and you’re willing to follow the author anywhere he or she wants to take you.
But you need to find a balance. This is why revising is such a helpful process. You might love that long flashback in Chapter 15 that reveals so much about your main character, but is all of it needed? Should that flashback be broken up into two or three sections and spread throughout the novel? Does the flashback need to be there at all?
You will get better at integrating backstory and flashbacks through lots and lots of practice. I’ve written nineteen novels, and I still struggle with this part. Too often I want to just totally focus on the current narrative, on what the character is discovering right now, on what the next scene has in store for everybody.
But you can’t only focus on the here and now in your narrative. Doing so robs the reader of the full experience of your story and characters.
It’s vital to find that balance of looking forward and looking backward in your fiction writing.
There’s not exactly a percentage to follow. Every short story and novel is different. Some novels, for example, are written almost entirely in flashback, and sometimes that works and sometimes that doesn’t.
But if I had to throw a couple of numbers out there, I would suggest that around 80% to 90% of your story should be focused on the current narrative and be looking forward, while 10% to 20% of your story should have elements of backstory and flashbacks.
I feel like those are solid estimates to go by. But again, a lot of it depends on your story, and your genre, too. And when it comes to short fiction, you might try to blow all this up by telling a story completely in flashback. You might even have a good reason for including five long chapters of flashback in a row in your latest novel.
But proceed in those extreme areas with caution. Be careful about spending too much time in the past in your fiction writing, especially in a novel, because you might alienate some or many of your readers.
Try to find a balance. Revise your book over and over to discover that perfect balance.
Doing so will allow you to eventually find success as a fiction writer!
brianrowebooks.com 
Great advice, here! I’ve seen stories and novels where the first few pages are full of flashbacks. I’m with you. I tend to put down those books and look for another one to read.
Thanks for reading, Jack!
Good advice