How long is it safe for a person to water fast for?

Hi,

I was wondering how long is it safe for me to fast for without causing my organs any internal damage. I am a 20 year old "reasonably healthy" female who weighs 120lb (with a fat content of about 23%). I want to fast for long length of time (10-20 days), but I am afraid to hurt myself by pushing it to far. What do you recommend? Would a 14/20 day fast be too much?

Thanks,

Carina Tweet