I find that around Day 3 or 4, the headaches hit. I never paid them much attention before since I was typically never going 3 or 4 days without alcohol, ha. But once I reached the point of drinking a bit less, I noticed it more. Alcohol was the only thing that was the missing element so I knew it was that.
I increased my water, took Ibuprofen if necessary and realized they typically went away after about Day 5.
I also think it may take a few days for the signs of dehydration to show up?