Poinsettia's fall under different response times....anywhere from 8 to 12 weeks for signs of first color. Once you begin seeing coloring in the bracts you can stop with the SD treatment.
As a graduate of greenhouse
/nursery all of us students were in charge of growing
our own crops of Poinsettia (3-4 different cultivars each) and we started our shortday treatment with the fall equinox (Sept. 22). The rooted cutting arrived from Fishers and Ecke (websites online for more info) in July. plants
were ready between Nov. 12th thru December 12th. Poinsettia require more Mg and Ca then most plants
and benefit from a feeding of a 15-5-15 Cal-Mag at least every other 3rd watering to prevent stems from becoming brittle or weak...eventually breaking off.
I have had my own personal plant
for 3 years (the current one a cutting from the mother plant
) and failed to feed it the extra ca/mg and lost some stems this year. Finally I cut it clear back leaving 4-5 nodes which are currently leafing out.