What are the factors that affect the length of an orbit?

Central accelerating force, in the case of an orbit this would be determined by the central mass.

semi-major Radius of the orbit.

eccentricity of the orbit.

The value of the Gravitational constant.

For a central mass reference these four are sufficient. If you consider a 'center of mass' reference however then the mass of the orbital is also required to determine the orbital characteristics.