Labor Force Statistics from the Current Population Survey page: http://www.bls.gov/cps/cpsaat08.htm
According to Wikipedia, "the Fair Labor Standards Act (FLSA) does not define full-time employment or part-time employment," However, according to People's World: "On October 24, 1940, the 40 hour work week went into effect under the FLSA. The new law had been signed by President Roosevelt in 1938."
My generation grew up with the assumption that full-time was 9am to 5pm Monday through Friday. At least until 1979 when the war on Unions and deregulation gained momentum. See these articles:
The Number of Salaried Workers Guaranteed Overtime Pay Has Plummeted Since 1979
The expanding role of temporary help services from 1990 to 2008, Bureau of Labor Statistics, Tian Luo, Amar Mann, and Richard Holden; August 2010
There Are A Lot Of Part-Time Workers In Post-Financial Crisis America, Business Insider, Doug Shourt, Advisor Perspectives; November 10, 2014 -- http://www.businessinsider.com/ratio-of-part-time-employed-remains-substantially-higher-than-the-pre-recession-level-2014-11
Why The 40-Hour Workweek Is Dying, Forbes, Jayson DeMers; May 15, 2015 -- http://www.forbes.com/sites/jaysondemers/2015/05/15/why-the-40-hour-workweek-is-dying/
The Full-Time Job Is Dead, BackChannel, Kevin Maney; June 4, 2015 -- https://medium.com/backchannel/the-full-time-job-is-dead-b9528bda1c87