According to the 495expresslanes.com website "Construction began in the summer of 2008. The Express Lanes were completed on November 17, 2012"
https://www.495expresslanes.com/project-background
I'll leave it to someone else to do your googling to find out how long the gainsville expansion took.
According to
http://www.virginiadot.org/newsroom/northern_virginia/2010/i-66_widening_nears_completion47247.asp
Construction ran 2006-2010.
Two lanes were added in each direction, not one. Making it the same number of lanes but without all the ramps/ bridges that were part of the beltway project.