The study of a single group of gifted students from the 1920s continued for their entire lives. The last big work was completed in 1986, took 9 years to code, and was published in 1995. They were still doing studies in the early 2000s with participants that remained.
The attrition rates are surprisingly small to me for something that continued for so long. To have about 1/2 still answering questionaires after 60+ years is amazing (they started when they were 11 years old or so)