I believe your answer is going to be 2 Weeks. Hope this helps!
I've had a friend similar to yours, yeah you can encourage them to eat but don't make them feel to bad about it. Just try to be there for them because there might be a reason behind why she isn't eating, you should maybe ask her about it. If it gets really serious you should go to an adult.
I hope everything gets better : )
Positive: eating healthy, drinking lots of water, stretching before working out
Negative: eating junk, drinking sugary drinks, not stretching before working out
