Here's something I found about the duration of immunity with the lepto vaccine.
"2. The commonly used vaccine only contains serovars Lepto. canicola, and Lepto icterohaemorrhagiae, and no cross protection is provided against the other three serovars diagnosed in Texas. Newer vaccines containing Lepto pomona, and Lepto rippotyphosa are available but the duration of immunity is less than one year. To provide protection for a dog against Leptospirosis would require two vaccines with four serovars twice per year".
From this link...
Texas Vet sues due to over vaccination of dogs and cats