Not Rain Fast.

Well, the pictures didn't turn out as well as I had hoped, but I could definitely see the difference between what I sprayed before and after the rain last time. Generally you need anywhere from 2 to 6 hours for the herbicide to be absorbed before getting a rain.
To the right is what was sprayed before the rain, so the herbicide got washed off of the plants before being properly absorbed. The cross piece is the exact spot where I stopped spraying to wait out the rain.