For how much time does rabies vaccine provide protection i think i was scratched by a stray dog?

Don't count on it: The protection after rabies vaccination gradually decreases, so even if you've been vaccinated a booster series of 2 doses is recommended if you have been exposed. That said, it is not clear that you have been exposed. Dogs are not as likely as other animals to have rabies, and rabies from scratches (nonbites) is rare, but i would check with your health department to estimate the risk in your area.

Wash First: Clean the scratch with soap and water for a good 5 minutes--then head to emergency room. Even vaccinated you may still need rabies vaccine, just not as many doses and just no rabies immune globulin. While dogs are less risky as rabies carriers they are making a comeback in some areas. This is a decision that will have to be made at a local level, with the involvement of your animal control office.