Maintain a low-calorie lifestyle?

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Hi All,

I'm currently doing a fast, and I've noticed that after a few days into it-it's hard for my body to take in food since it's gotten used to minimal eating. I eat/drink very early before morning daylight, and don't eat again until the evening around 8:30pm. So, on some days I can only handle eating 600 calories for that day.
I currently weigh 203lbs, 5'5
I'm trying to reach my weight goal of 140-150lbs

Question: For how much longer can I maintain a 600-1000 tops calorie intake to lose weight, without the weight coming back?