A lot of posts in here say that the bottom line is fuel..... not true.
The bottom line is that once airborne the aircraft must remain within the specified flight time (i.e. 120 min or 180mins etc) in still air at the proscribed speed (varies for each airplane and company) of an airfield that the airplane can land at.
Prior to departure there are a number of requirements regarding fuel, weather conditions and aerodrome suitability that must be met but they disappear once airborne where the first paragraph become the only requirement.
