A complex airplane is defined by 14 CFR 61.31(e) as an airplane that is equipped with bit not limited to the following:
Retractable landing gear
Controllable pitch propeller
Before August 1997 special requirements to operate complex airplanes didn't exist under part 61 and was considered to be part of high-performance criteria. Today a separate endorsement is required for each of those operating privileges.
There is no minimum number of flight hours required to complete a complex endorsement, although you will be required to receive and log ground and flight training from an authorized instructor in a complex airplane and need to be found proficient in the operation and systems of the airplane.