41 days from seed

When should I flip the lights to 12-12 my plants is 41 days old from seed

When you think the plant is ready.

To get a better answer, you'd do well to share:
Pics (this, if nothing else)
What, if any, training you've been doing
Strain

Others might also want to know things like what exact light, soil mixture, feeding schedule, and such, but for this that's probably overkill.

As far as I can tell, there's no hard & fast rule about when to switch to 12/12, beyond when the plant is ready, and if it's ready has so many "it depends" on it that it's hard to just blanket say. To me, once my chosen amount of colas are trained to where I want them, equal in height, and the plant is solid (healthy), that's when I flip. That said, I use tents, so I also have to keep an eye on canopy height. The plant will grow 1 to 1.5 times again in flower, so that's another factor you can use to decide when to flip; when it's 1.5 times short of where you want to finish.

When ever you want

You see preflowers yet?
I wait to see them before flip, makes transition to flower faster if they are mature and ready.

Or you'll just have a longer flower period if you flip to early.

Flip a coin..

Got pics? How big is plant ? What light you using to veg/ flower? Size of room? Height restrictions? How many plants?
So many questions to decide on... when to flip...

Any alternating nodes to indicate sexual maturity?

Ok soil is pro mix with some added ewc and happy frog 7-4-5 in a 3 gallon smart pot watering every 2 days or when pot feels light and I have topped her once and started some lst as well

And it’s a female plant bought the seed feminized

Oh and the stain is bad azz og cheese

They are not sexual mature yet.