How long does it take to go in starvation mode?

About 1 to 2 days, but in that period of time you will mainly just lose water weight and could continue to be just water weight for up to 4 days. By 7 days your body will be fully in the starvation mode as in everything you eat will become fat and your body will begin to grow hair all over (even on your back) I believe the medical term for it is lanugo.