For how long is the tetanus vaccine effective?

I am not completely sure, whether I have undergone a complete dosage of anti-tetanus vaccine in childhood. But I am sure about 3 doses, which I took from my doctor. They are first when I was 17 years old. Second, when I was 25 years old and third a week back. How long will it be effective? Some doctors say that tetanus vaccine is effective for 6 months and some say 5 years. Please elaborate.