Starvation Mode

How does the body decide when it goes into starvation mode?  And how long does it last?

It seems to be mentioned everywhere, but I don't really understand the details of it ._.

Thanks 

It's a very complex biological process. It's in essence a defense mechanism against starvation.

From my understanding, what happens is your body will begin to consume more muscle than fat for energy if you eat too little. It does this because muscle requires more energy than fat to maintain; this has the added affect of lowering your overall metabolism, simply because your body, having less muscle mass, needs less energy to stay alive. All calorie restricting diets will have this affect to some degree, but you can keep it from occurring too dramatically by making sure you get adequate calories, nutrition and exercise.

There is no formula for how long it takes for your body to go from consuming mostly fat for energy to consuming mostly muscle, or vice versa, but a good rule of thumb is to never have more than a 1,000 Calorie deficit. You have to just keep in touch with your body's changes. Presumably one big clue to starvation mode is lethargy & fatigue.

edit: added more information