How long can I use glutathione pills and vitamin C to lighten skin?

I have a dark brown skin (similar to Kelly Rowland) and I would like to lighten it by 1-2 shades. I was advised to take glutathione pills with vitamin c. Recommend dosage is 1000mg of glutathione pills daily but I don't want to become too light. I just want to be 1-2 shades lighter. How much of glutathione and vitamin c should i take to achieve my desired shade?

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