Up to 8 or 10 feet away from a south, east or west facing window. Intense direct sun can burn plant leaves for all but the most hearty cacti. Note that morning sun from an east facing window is less harsh than afternoon sun from a west facing window.
Bright, filtered light: Direct sun but filtered through sheer curtains or similar non-solid screening.
Partial sun: Direct sun for a few hours a day, best from east or west facing windows.
Little natural light and no direct sun at all. Can be a north facing window, or a window facing the sun but is blocked by a building or other large obstruction. Note that no plant, even low light plants, will thrive with little to no light.