Combine the 2006 season, when Romo took over as starter for the remaining 10 games - and 2010, when Romo started the first 6 games before being injured - 2006+2010 combined = one full season as the starting QB.
2007-09, then 2011-14 are 7 full seasons when Romo was the starting QB from basically the beginning to the end (out 3 games in 2008, 1 each in 2013 and 2014).
So really Romo tallied 8 seasons as the Cowboys starter, with 2 seasons missed for injuries...2015 and 2016.
127 regular season starts in Romo's career ÷ 16-game regular season schedule...averages out to 8 full seasons (7.94).