Quotes from Wild Lettuce wiki draft
The whole plant is rich in a milky bitter sap that flows freely from any wounds. This hardens and dries when in contact with the air. It is harvested in the summer when it is in flower. That's when the plant puts its energy in mating. How long it will live depends on your growing season.
Now, depending on the source, "the flowering period falls in June-August" or "it is in flower from July to September". Can't tell which one is closer to the truth. The source for the first possiblity looks quite reliable (A. Stojakowska, J.Malarz, W. Kisiel “Lactuca virosa: in vitro culture and production of sesquiterpene lactones” in Medicinal and aromatic plants, Volume 11 by Y. P. S. Baja), the other one comes from an unidentified website. Could this be the flowering period is dependent on the climate in which given specimen grows? Can this be dependent on some other factors? I'm not a botanist and I can't recall ever seening this plant in nature.
Concentrations of lactucarium are low in young plants and most concentrated when the plant comes into flower. It is collected commercially by cutting the heads of the plants and scraping the juice into china vessels several times a day until the plant is exhausted. The liquid from the stem is the most potent part of the plant.
This is all I've found so far while collecting data to be included in Lactuca wiki.
Unfortunately can't tell anything about the practice of collecting lactucarium either but if you check the wiki article
, there is a close-up photo of a cut stem oozing the sap.