When should i stop vegging these photoperiods to start blooming? I don’t want them too tall because this LED light hangs lower than expected

You can flip now if you'd like, but if you want a bigger yield, you can veg longer. Just keep in mind many plants will stretch 2x their height (some 3x) after flipping to a flowering light cycle.

When your plant reaches half the max hanging height of your light you NEED to flip. Any point before that you can also get good bud but don't exactly have to flip. It is more or less up to you and what you want to get out of it. Basically, the choice is yours until it reaches that half way point.