I have tried the store brands, Frontline, Frontline Plus, and Advantage. By far, the Frontline Plus works best. Sometimes we would go several months between treatments and it was still effective. Look at the box, it says it lasts 3 months for fleas and 1 month for ticks. Our dog was allergic to fleas and once we started the Frontline Plus, within a few days, he stopped itching and his rash went away a few weeks later.
If you purchase from your vet, it will be fairly expensive. However, there are several places online that you can find some good deals, especially ebay. Ask your vet...