I-65 does most of the work between Franklin and Nashville, and that single road is why “Franklin to Nashville” isn’t one commute time. It’s a range that depends on which Franklin exit you start from, what time you leave, and whether your destination is downtown, the airport side, the west side or an office park that never requires you to enter the city at all.
Two people can both live in Franklin, work “in Nashville,” and have drives that don’t resemble each other. Within Franklin itself, getting from the south end to Cool Springs at 8am is its own trip.
Test the drive you’d actually make, at the time you’d actually make it. That number is the only one worth trusting.
I won’t quote you a fixed drive time, because construction, growth and the hour of day change it constantly. What I will do is tell you which side of Franklin makes your specific week easier.