In Lights palette double click on your light to invoke Light Parameters dialog.
In the Intensity tab set Falloff to Linear Clamped. Note, Ambient, Distant and Environment lights don’t have a Falloff option.
The falloff distance for Point, Area, Line, and Custom lights is set by the Radius distance in the Parameters tab.
The falloff distance for Cone and Projector lights is set by the distance between the Origin and the Center of Interest. To change this distance click the 2nd column next to the light name in the Lights palette to display the light in the modeling window then Right click on it and choose Show Controls. Now drag the Center of Interest arrow towards or away from the Origin to increase or decrease how fast the light gets dimmer.
In addition, for Cone lights you can also drag to separate the Outer and Inner Angle to achieve a softer falloff between the inner and outer cone of light.