I'm writing a full length novel in which the main setting would be a city. Generally, how long of a backstory should I implement before the two main characters reach it?
Get your characters going on page 1. Introduce any necessary information about the city as you need to, as you go along. But get the reader engaged with the characters as soon as you can. It's generally a mistake to start with pages of backstory describing your city before you introduce your characters and get them moving on their story.
I'm going to second what Minstrel said. Start your story and bring in the back story as you go along.
Zero words. Don't start with back story. Don't write back story at all, write story. If a tidbit of background information is needed at all, introduce it as late as possible, and leave out the fluff. Background is better off left in the shadows, silent and mysterious.
Just agreeing with everyone else: Zero backstory. In fact, I'd consider whether the characters' arrival is interesting enough, or if your story should start after they've arrived.
Start your story with the interesting thing the story is about - don't take the long runway approach, with pages and pages of 'I'm about to get to the really interesting bit, but first I just need to tell you.....' Cut to the chase.
Despite what the other posts have said - I'm not sure I understand what you are actually asking? Are you asking - How much of a prologue do I need to write to explain how the characters arrived at the city? or - How much of a character prologue do I need to write to explain why they are at the city? or - How long do I make the story that leads them to the city? I'm just trying to be accurate - hope those distinctions make sense. I see no problem in writing the story that leads the two characters to the city and having that meeting become a larger story - but it must be a story and not just a long winded introduction. I'm not so sure about prologues though....I'm torn between their usefulness or lack thereof tbh. I've read some books where I enjoyed the prologue and others where I just found it boring and mostly useless.
I agree with minstrel, plus i also didn't get what you meant by the characters reaching it? The city? or the start of the story? In either case, no backstory, do as the others have said and add necessary information as you go.
If you're doing a fantasy especially, the backstory should be detailed, but as others have said, the readers don't need to know it all straight away. Just gently feed in the important stuff as you go. Having said that I don't think there's anything wrong with having little trivia facts about your backstory, providing it's not an info-dump. It makes the world feel more alive and makes things more challenging for the reader, as they have to decipher out what is important and what's just a fun fact.
I think I didn't phrase this properly. The couple in my novel will eventually reach a city; this is where the majority of the story will take place. Before the move, they have to leave their respective families, ect. I know there is no concrete answer to this question because it is dependent on my vision, but generally, how long should I extend this sort of "opening" before diving into the main front of the story. "excuse my lack of writing terms"
The best thing to do is to introduce and explain things as your characters come into interaction terms with that object. For example, they enter the city, give their first impression to the reader. Don't talk about the history of it just yet, when the characters get in touch with the citizens there, throw in a neighbour who gives some information (for example). Next, they go to the grocery shop, throw in some information here too. Since them leaving their families is sort of a back story, make it a prologue or turn it into a discussion between the two as they are driving towards that city. Just my opinion.
The question of "how long" is flawed. What you should ask is: "Is this necessary?" As long as everything you write contributes to the overall story in a significant way, that this is necessary for the story to progress, then write as much as you like. On the other hand if you're gonna spend entire chapters describing how your MC cooks soup and feeds his rabbits and then gaze into the horizon, and then spend another few chapters having him ride a trotting horse while he does nothing and you describe nothing but sun and grass - then skip the whole shabang. However, if you're spending chapters talking about how your MC makes soup for his mother because his relationship with his mother is gonna be very significant later on in the story, then it's worth spending a few pages at least on it, even if it's just making soup. So it depends. But there's no exact length you should go for. As yourself, if you didn't know the story, you just picked this up off the shelf and you have no idea who J.R Nayman is or what "The Voyage to the Starygatrop" is, and you just flicked open a random page and read what you wrote, would you be interested? Or better still, would you care?