lighting cycle...

i'm growing indoors in soil and its my first plant. how would i tell when i should switch the lighting to 12 on 12 off? any help would be great

How much space do you have? Keep in mind that, depending on the strain, plants can double in size during flowering.

i have it in my closet that is 3x4 feet approximately. its also about 6-7 weeks old and i don't know that strain it is.

You can switch the light cycle whenever you please,remember like stated above that plants double in size........so it's really up to you

As the others have said, expect the plant to at least double in size during flowering. You can switch to flowering as soon as you like but it is probably best that before flowering the plant that it has had at least four weeks vegging. You dont have to but it does help the plant if its mature enough to be handling the nutrients you want to be feeding it so four weeks vegging is good to slowly build up the amount of nutes you use, dont want nute burn after all.