Some weeds can show signs of discoloration or curling in a day or two, while others can take a week or two. Weeds usually die faster in warmer weather. Some tricky weeds may take a repeat application.
Many of the herbicides we use are rain-fast within 2-4 hours. We always combine quality surfactants which help the products stick to the target weed. We try to plan applications well enough before a rain that they are effective.
Every treatment is a step in the right direction and it can be a process. A highly weed infested lawn can take repeat applications to effectively control them. While weeds are being eliminated, desirable turfgrass is slowly filling in until an improved lawn takes shape. This process can take 6 months to a year to have a really good looking lawn.