In my case with my first round of cilantro smoothies it took about 3 weeks to have hope that my outbreak was stalling. My cycle pattern after my first full one year of severe front and back torso papuales, was 6 months of dormancy and 6 months of activity. I was in my 3rd year of dealing with this when another outbreak started. Cilantro stalled the outbreak and dimminished the itching and pain, yet it took over 6 months for the crusted papuales to fall off , they just hung there and if scratched they would be replaced, just mildly itchy. I was confident it was working after 3 months, yet out of fear I continued the blended cilantro 6 months, probably longer than anyone else on this site. Then I had 2 years of dormancy, doing anything I wanted- sweating in the hot summer garden, other activities like rough off roading that create intense friction on the back and no issues and then in Nov of 21 noticed it was beginning again. It wasn't as severe as I was used to but finally decided to back on the smoothies in January. My recurrance just stayed mild, yet I kept up the smoothies until a few weeks ago as I now think this outbreak is over, yet I still a few inactive papulaes hanging on, but no itching. The tingling and pain are gone. I've even resumed some activites that in the past have aggrevated it.
I no longer put the cilantro in my delicious morning smoothie. I simply put half a bunch, fresh or frozen, stems and pieces into a blender with about 1/2 cup water, blend and then hold my breath and drink it down, considering it my morning tonic. I hold my breathe and the taste is not noticeable and it doesn't contaminate the taste of my regular health smoothie.
Best of luck, it's a treament that has helped many, but not all. Not one other treatment prescribed by doctors ever gave me relief or shortened my outbreaks. I read every eczema site I could find on the internet, including the Mayo skin site and learned nothing of any personal benefit about GD. I have a medicine cabinet full of expense useless prescriptions. This site and sweet Kimass1 who recommended cilantro gave me my life back.

It depends on the individual. Different times for different people.