And don't forget, even Elon Musk can't change the laws of physics: to propel an object into the air will always take the same number of electrons and it takes time and infrastructure to put electrons into batteries, even if there is a quantum leap in energy density of batteries.
With a single onboard charger on a Model S Tesla, being charged from a normal (US) 110V outlet, you get 5 miles of range for every hour of charging. From 0 to 300 miles of range takes 52 hours.
Given the inevitable safety margins required for an electrically powered helicopter, I cannot see it ever being a practical proposition.