Switching Light Times

when my plant reach the size i want do i just change over to 12-12 to start flowering or do i have to wait a certain time this is my first grow so im not sure

Switch it right over when the plants are the size you want them to be.

thanks alot

No, switch it over right before the size you want.
All plants stretch during early flowering.

Especially sativas.

What strain are you using?
What kind of lighting?
How old are they?

Most healthy plants will stretch between 50% -> 200% of their size once you flower.
In other words, a 12" plant will stretch to somewhere between 18"-36" before harvest.

nnot sure what strain i found the seeds in my kids room after he moved out,he said they were donkey dick and ducks foot but i have
4 plants and they all look different and i am using 3 48" florecent lights and they are about 2 1/2 months old but they are about the size i want them amd they all look extremly healthy

I would like to encourage you to take a look at Compact Fluorescent Lamps. "CFL".
Way more efficient, better color spectrums, and easier to use.
Go ahead and flower them, but be advised they will probably shoot up at least 6" in the first week and a half.