About 911 nm — roughly 2h 31m in a midsize jet. 2 legs live on this route right now.
Every price on EmpTleg buys the entire aircraft, not a seat.
Miami to San Juan is a different flight with its own aircraft, its own days and its own price: empty legs from Miami to San Juan.
Nobody schedules an empty leg. One shows up here because an operator has an aircraft finishing a trip near San Juan and needs it near Miami — 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.