I can't give beat-for-beat answers to your questions, because honestly, I don't know. When I get an idea, I figure out what's necessary to know to pull it off, usually scaffolding myself towards the scene I want to write--in the case of you only get one night up on the shore, it was Gojo sucking off Yuuki's prehensile clit. Details like dates and time spans don't factor in unless they're an important aspect of the fic

With that disclaimer out of the way: My original version of the fic, before I adjusted it to fit what I needed for the Spite Project, would have started from Gojo's and Yuuki's first meeting and onward, however far I'd have taken that. Of course, as anyone who's read the fic can tell, that plan changed. The fic instead features a version of goyuu who are not only already together, but comfortably so. But that previous version of the idea means I can sketch out some vague answers to your questions

It's mentioned in the fic, but Gojo likes to grab curses to gift to Shoko so she can dissect them. It's part of why he knows so much about the appearance of curses depending on where they spawn, city vs mountain vs ocean. That's how Yuuki and Gojo first meet, because Yuuki goes to investigate the weirdo stealing her food and Gojo, because he's insane, doesn't exorcise her on sight or drag her back to Shoko as his curse of the day. Somewhere in my scraps doc for this idea, I actually have that scene mostly written out

I have no idea how long they've been together. I implied it's been a while, with Gojo thinking and saying that Yuuki was smaller when they met, but even I have no clue when that was. The fic takes place in 2018, exact month unknown, but in my mind, they likely met before the Night Parade of a Hundred Demons. Their chemistry when they meet would carry them from there, I think. They just click, same as they do in canon

As for how many people Yuuki's snacked on, the answer is, I think, so fucking many. I couldn't showcase it much in the fic, just because there wasn't space to fit it, but Yuuki sees everyone--even Gojo--as food. If I'd been writing a longer fic, I'd have dug into that aspect of their relationship so much. Gojo's a sorcerer practically from birth, raised to exorcise curses, but he's so strong that it never tips into self-defence territory. There's no instinct to it, for him. Yuuki, on the other hand, is Kenny's special little experiment, and she's lived in the ocean her whole life. Everything that doesn't flee from her wants to eat her. It can't, obviously, but she's used to existing in an environment where everything around her is edible. And she's not a human nor a curse, so when she decides to see what all this land stuff is about, she's visible to humans. Humans, historically, do not react well to things that activate their uncanny valley response--there's a reason, besides the excuse to get naked, that Gojo teaches Yuuki how to better mimic a human appearance

The end result of all that is a creature that has never been raised with any kind of morals, existing in a space where nothing can eat it but everything wants to, who can eat whatever it wants whenever it's hungry and has never had to examine this fact at all. The only saving grace, besides Yuuki having to keep some of her canonical traits or she'd cease to be Yuuki at all, is that she's curious enough about what's on land that she works to blend in

And she succeeds, eventually!! Gojo gets to admire her perfect ankles!! But like her two-legged body, it's a mask she's choosing to put on. Internally, humans are kebabs on legs to her. She's not going out of her way to eat them, but there's no instinctive screaming voice in her head that says cannibalism is bad, because one, she's constantly hungry, and two, she's not human. Even after she likes Gojo, she still takes chunks out of him--see Gojo remembering that time she bit his throat out for putting his fingers in her gills, and also the fact that those two fuck nasty in every universe

Basically: She ate a metric fuckton of people before she met Gojo, who would make sure she knew immediately not to eat any of his cute little students no matter how edible they might be. But Gojo is, at heart, also an asshole. He sets no more limits than that, and Yuuki never wonders if he should. Yuuki doesn't have a need, really, to eat humans specifically, but if she's pushed past a point, she'll kill them, and if she has, it's a waste of meat to just leave it there, right? She's a growing sea monster, she needs the calories. But she's just as happy to get those calories from curses or ordinary food and the chunks she takes out of Gojo