Most multirotors with a modest payload will find 15 minutes to be an achievable target with the right size battery. Longer flights become exponentially harder for a given payload. In theory large, slow rotors are inferior for control, but superior for efficiency. The size of a helicopter's rotor makes it potentially a lot more efficient, but the fact that it has to be variable pitch cuts it back down.
Competitive endurance runs tend to be in the 45-90 minute range