Its more dependent on the age, not height. So, how long does a plant veg for, then how long it should be in 12/12.
CYCLE TIMES
Given the above data it can take anywhere between 10 weeks
and 36 weeks for a plant to grow to maturity. That is, again, anywhere
between 2.5 months to 9 months. The most common grow time is 3 to
4 months. All this is dependent on the strain that you choose. Pure
Sativa can run anywhere into the 6 - 9 month bracket. Indica can
flower in 6 weeks. As you can image a Sativa/Indica plant will fall into
the 2 - 4 month flowering period.