Generally speaking, the brighter the light, the less time it takes to use.
With the most efficient lights (e.g. powerful SAD lightboxes designed to provide 10,000lux at arm's length) you only need around 30 minutes or less each day. LED devices with blue-enriched white light are also quick to use.
Mid-power lights (around 5000lux at arm's length) take around 1 hour and appeal to a wide range of users.
If you can spend longer in front of a lightbox (e.g. while you're at your desk) or you can dip in and out throughout the day, the lowest power 2500lux lights typically take 2 hours and are a very economic alternative.
Everyone's different so, to start with, we advise you to use your light for slightly longer than stated in the instructions and see how you get on. After a few days, if your symptoms have improved, you could gradually cut down and see if you notice any difference.