People vary in how much bright light they need to combat depressive symptoms. There must be many factors, including how depressed a person is. A modest increase in lighting may help with a mild problem. A more severe problem may need much brighter lighting and a longer duration of bright lighting to overcome. The amounts of light needed should at least bring a depressed person above the average for daily light exposure. This can be achieved with as little as 30 minutes of very bright light near 10,000 lux (like sunshine) or with a couple hours of light of about 2500 lux (like a cloudy day or deep shade).
It is best to use 10000 lux light when using light treatment. If you use 10000 lux, you can get benefits in around 30 minutes whereas a light box that emits 2500 lux might take 2 hours to get benefits.