What do you do, If a customer complains that weeds are still growing in the lawn, shortly after you applied herbicide?
In my paper work that I leave for the customers explains that it can take up to 30 days to see physical changes to the weeds treated.
Really depends on timing. If they are calling day after, tell them to get panties out of a bunch and give it a week or two If 7-14 days after, go take a look and retreat as you see fit. Over 4 weeks, hell no, charge for another app way too long after previous app.
The customer is always right. Spray it again--at least spot spray, and within 24 hours. Better in the long run. Customer satisfaction and loyalty go a long way.