In a immuno-competent person, immunized by good quality modern anti-rabies vaccine, immunity lasts for more than 10 years.
Still, if dog bites again, even after 3 months, it is advised to take booster doses (2 only) on day 0 and 3. This is meant to take no chance looking in to the irreversibility of the illness, if that happens. In this situation, RIG (Rabies Immuno Globin) will not be required.