I-65 does most of the work, and the local roads decide how long it takes you to reach it. Old Hickory Boulevard, Franklin Road, Concord, Moores Lane and the Nolensville Road side of the county all feed traffic differently, so “Brentwood to Nashville” isn’t one number.
Someone working downtown, someone working in Cool Springs, someone headed toward the airport side, someone going to West Nashville and someone commuting into Rutherford County are solving completely different problems from the same city — and sometimes from the same street.
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 — construction, growth and the hour of day change it constantly. What I will do is tell you which side of Brentwood makes your specific week easier.