Frontline contains a powerful insecticide that begins to kill ticks on contact. Fleas are killed within 12 hours of contact whereas ticks take about 36 hours to die.
The medicine only kills adult ticks that come in contact with your pet as they seek out a blood meal. As the adults are killed, more developing ticks will seek out your pet for food and be killed as well.
It may take a few months for the entire population of ticks to be eradicated as all stages of the lifecycle must be killed, but you will see a vast reduction in the amount of ticks on your animal within hours after initial application.