Nice to meet you @Darth_Biomech
There's no true rule I know of,
I guess the character's mood and intensity level of the scene determines the manner a flashback transition occurs. A fast paced action scene or important scene is better with a very short flashback because the viewer would be heavily invested in the 'present scene', you wouldn't want to cut away too long to frustrate them.
But if they got K.O.ed in that 'present scene' like you mentioned, you can go a bit longer with the flashback to show hours have passed when they do wake up in the present.
Having a character's eyesight lock onto an Object within the environment (person, prop, or something that stimulates the 5 senses) can also be a neat way to trigger a memory. This object either (a) is significant to his/her past or (b) reminds him/her of something from the past:
eg scene. Mc is sitting in a café waiting for her friend, she sees a little girl with a yellow balloon sitting on a nearby table. We see the Mc's eyes widen as she stares at the child with shock, we then we see a closeup of the yellow balloon (the object she was actually staring at). It cuts to a flashback of her terrifying past, where her little sister holding a similar yellow balloon was killed by a reckless driver
[In the above scene it also portrays 'Show don't tell' eg. we see that the yellow balloon gives the above mc traumatic memories and possibly spacing out. Having her say 'Omg I hate yellow balloons' can come across cheesy, but her facial expression and memories are enough to portray her personal struggle to the viewer.]
eg scene 2. During a major battle, the Mc is struck by his rival with immense force! The necklace Mc was wearing brakes apart and falls beside his weakened body now slumped onto the floor. The Mc quickly glances at the broken necklace (closeup of broken necklace). We transition to a flashback where his marital arts master gifted him that necklace for his hard training and advised him to never give up.
[In this scenario the viewer would be eager for mc to get up and kick his rival butt, but what was the level of damage inflicted onto Mc? Did it stun him for 5 seconds, or 5 hours? The length of the flashback can be used to reflect that.
It would feel very weird if the flashback dragged on for an entire episode+ and then we cut back to the Rival still standing there, the Mc only received a scratch wound n quickly returns to his feet.]
A flashback's purpose is to further aid the viewer in understanding the overall plot, and get into the character's head: based on his/her reception to their current environment.
Put yourself in the viewer's shoes, how do you want them to feel when they read your chapters? The guys who replied above said some good stuff, and yea you never want to give away everything in one scene. You want to keep the viewer curious to find out what happens next so they stay till the end. Always keep the scenes n flashbacks to the point, you never want to deviate far from the main plot or you'll throw viewers off. There's something called 'red herrings' don't abuse it, but they are neat ways to trick your viewers and mislead your Mc on his/her journey, they can help you avoid making the plot predictable.
https://www.masterclass.com/articles/what-is-a-red-herring-in-writing-definition-of-red-herring-with-examples
I sort of wrote this unplanned, but hope it serves you some use
Wishing you success on your creative journey! ![:star: :star:](https://forums.tapas.io/images/emoji/emoji_one/star.png?v=3)