5-10 years. You recieve doses of dtap at 2mo, 4mo, 6mo and then 15mo. Then a booster again at 4 years of age. Then another at age 11. And then every 5-10 years (according to some experts). If fully immunized.
Differing times. Pertussis usually does not last more than 8 years. Tetanus can last for a very long time. I have checked immunity for tetanus 25 years after the last vaccine and still found very strong immunity. It is best to check for immunity before repeating any vaccine. There is no need to repeat if the immunity is there. There are so many toxins in vaccines so if you have to vaccinate, do as few as possible.