When does THC first start to form?

THC is present in the leaves and stems of the plant from a very early age...
But the good shit THC is produced during flowering.
To be more precise, about 1 weeks into flowering when you can determine the plants sex.
That heavenly funk you smell during flowering is THC being made.

 

if you are going with one month of 24 and two months 12. you could sample a bud about week eight, when it is half way through buding. thats just my opinion and could be wrong.