When you have an eating disorder, how long does it take to lose weight?

THIS IS NOT ME! I am asking because I am very worried about my cousin, she recently lost weight, and some of her behavior towards food is suspicious. so I was wondering how long it would take, to see if that's possibly what she is doing. and to be able to talk to her about it.
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I want to know if my suspicions are correct.
When you have an eating disorder, how long does it take to lose weight?
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