About 285 nm — roughly 1h 02m in a midsize jet. 2 legs live on this route right now.
Every price on EmpTleg buys the entire aircraft, not a seat.
Jacksonville to Miami is a different flight with its own aircraft, its own days and its own price: empty legs from Jacksonville to Miami.
Nobody schedules an empty leg. One shows up here because an operator has an aircraft finishing a trip near Miami and needs it near Jacksonville — or needs it home, and this direction happens to be the way home. That is why the flight can be a fraction of a full charter, why it can vanish within hours, and why the return direction is a different flight entirely with its own price and its own aircraft. Watching the route beats searching it once.