Hey!
General rules for defoliation are taking away the biggest fan leaves with the long stems, the ones that give the most shade.
Then you can take out all the under growth on the bottom that simply won't amount to anything.
You want to take a good 20% off, enough to give more light penetration but where its still a bit hard to see through you plants.
This is done when you switch to 12/12..
Three weeks in to flower you do another defoliation. applying the same rules.
After that they can normally go untouched except for plucking a fan leaf here or there thats blocking major bud sites.
If you want visual, I recently posted a bunch of photos and videos of before and after defoliation on my diaries.
Will be posting the 3 weeks before and after defoliation this week.
If you want an in depth tutorial check this out.
https://www.growweedeasy.com/defoliation
Hope this helped.
Good luck and happy growing!