It typically takes anywhere from 24 to 48 hours for a dog dewormer to be effective after it has been administered. At this point in time, the roundworms should begin to pass through their system. If little white worms begin appearing in their stool one week after the dog dewormer has been administered, then it is an indication of the presence of fleas, as the little white worms are tapeworms.
Generally speaking, puppies are treated every two to three weeks with dog dewormers as they are most vulnerable and susceptible. Dog dewormers are typically mixed in with the food for several days.