The rule of thumb for any gun and any cartridge is 50fps for each inch of barrel. When I looked into this last year for a .308 semi-auto, the number seems closer to 30fps. After some extensive number crunching, I realized that a 16" .308 COULD, with the right load, reach out to 800 yards (825 if I remember correctly) without going subsonic, which is important if you like accuracy.
A 24" barrel will get you to 1,000 yards.
I wound up with an 18" barrel due to the gas system on the AR-10 needing to be at a certain threshold to remain consistent.