It's your preference. The whitening effects are permanent only if you avoid prolonged exposure to sunlight and maintain it. You will not go back to your original skin color even if you stop using glutathione; however, there is a tendency for you to darken because of sunlight and other harsh environmental elements.
It is also important to note that glutathione is not only a whitening agent, but is very good for our health. Being an excellent antioxidant, it improves the immune system and helps in preventing serious problems, such as cancer. Without the protection of glutathione, cells die at a faster rate, making you age quicker and putting your body at risk for toxin-induced diseases, especially liver cancer.