It really depends why you use light therapy. If you work in a dimly lit office, for example, you'll probably use your light every working day to make up for the lack of natural light. If you suffer with SAD you'll probably have a daily session in front of the light, maybe even two a day during the darkest part of winter. Some people can skip two or three days of light therapy whilst others notice a difference (as do their families, friends and colleagues!) if they miss even one day.
Even in winter, there can be days when it's bright outside and if you can get out and make the most of it you might not need extra light therapy that day and as spring comes around and the days get consistently brighter, you'll probably find you need to use your light less and less.
Dawn simulators help to regulate your sleep/wake cycle and are therefore best used every day, all year round.