Beginning with FDR and the 73rd United States Congress (March 4th, 1933) Democrats controlled both houses of Congress for 7 straight 2 years terms, up to the 79th Congress, in (January 3rd, 1947) and again for the 81st and 82nd Congress (January 3, 1949 – January 3, 1953) and again from the 84th Congress (January 3, 1955) through the 96th Congress, until January 3rd, 1981 and have controlled both houses simultaneously for another 6-2 year terms since, and have controlled at least one of the two houses, another 6 terms.
In 43 Sessions of the United States Congress, dating from March 4th, 1933 through today
Both Houses – Democrat 28 terms
Both Houses – Republican 9 terms
Senate Majority – Democrat 3 terms
Senate Majority – Republican 3 terms
House Majority – Democrat 3 terms
House Majority – Republican 1 term
With some minor +/-% modifications in a few sessions of Congress, the Democrats took control of both houses of Congress and the Oval Office on March 4th, 1933, with the election of Franklin Delano Roosevelt, who was President until his death in office, on April 12, 1945.
Democrats maintained control over both houses for the entire 12 years FDR was in office, and for another 2 years into the succeeding Truman presidency.
Though Republicans did resume control of both houses from January 3, 1947 – January 3, 1949, the Democrats again regained both houses from January 3, 1949 until January 3, 1953, with Republicans taking both houses again for the next 2 years, January 3, 1953 – January 3, 1955, only to lose both houses again to the Democrats, on January 3, 1955, who then retained control over both houses for the next 26 years straight, until January 3, 1981, when Republicans took a simple majority in the Senate.
So from March 4th, 1933 to January 3, 1981; with the exception of 2 separated 2 year sessions, Democrats controlled both houses of Congress for 44 of 48 years.
Since January 3, 1981, Democrats have controlled both houses in 4 continuous sessions from January 3, 1987 – January 3, 1995, again for 2 continuous sessions from January 3rd, 2007 – January 3rd, 2011, with Republicans controlling both houses for 3 continuous sessions from January 3, 1995 – January 3, 2001, then again for another 2 continuous sessions from January 3, 2003 – January 3, 2007, then regaining both houses again since January 3, 2015 to this writing, with all other years/sessions being split on differing majorities between both parties and both congressional houses.
In the total time frame from FDR to today, Democrats have controlled both houses of Congress for 56 of the last 85 years.