Lux is a measure of the intensity of visible light received by the subject at a given point. Since light is brightest and strongest at its source and becomes less intense with distance, the lux level stated for any lightbox is only meaningful if it also says at what distance.
Brazil, our most powerful lightbox easily provides 10,000lux at 35cm; smaller lights are less bright and to receive 10,000lux you'd need to have them closer; since that's not always practical or comfortable you simply use them for longer instead.
The initial research carried out into the treatment of SAD (seasonal affective disorder) by Dr. Norman Rosenthal used lightboxes that provided subjects with 10,000lux and this measurement is often still used as the standard for both effective light therapy and when comparing lightbox performance. We now know from recent research that other factors, such as the wavelength of light emitted, are also important.