Better to go by Trichrome development than by the calender. Depending on any number of factors even quick finishing plants can take longer than expected. Eight weeks is on the short side to begin with, especially since you don't know what you're dealing with. A lot of people wait for the Trichs to be 50/50 (Cloudy/Rusty).

But to answer your question, generally 8 weeks of flowering would mean from the onset of 12/12 (or whatever flowering cycle you happen to be using).

(Shake beat me to the punch)