From Dennis Lynch:

A standard unit used to compare one's active fannish life to Rusty Hevelin's. While Rusty was alive, it was a sliding scale, but with his passing, a full rusty is now defined as 70 years, the number of years that he attended cons, from 1941-2011.

Your fan life is measured as a portion of rusties. Divide the years you have been going to cons by 70. In my case, It is now 38 years, so 38/70 = 0.54 rusties.

See also, standlee, scalzi.