Starvation mode ?

It's a gradual process. It isn't easy to recover from either.

I read an article a few months ago that said about 3 months. No idea of its actually accurate, I can't remember the source.

From what I have read in the past:

About three days to a week of continual severe calorie deficiencies will trigger a famine response (or starvation mode) in your body. You should know if are about to enter starvation mode when you cease being hungry at all, because that is your body recognizing that there is no food to be eaten, and beginning it's metabolic slowing. This could slow down the metabolic rate up to 40%.

I should also mention that while starving, your body will start to use it's own resources for sustenance, however, it is physically impossible to continually be severely calorie deficient and gain weight. The weight gain happens when someone enters and exits starvation mode many times, triggering their body to store as much as it possibly can during the binge periods to replace your own materials that your body used while starving, while never really gaining back their original metabolic rate.

The best thing, and the healthiest thing to do is to do your best to eat regularly and as close you your goals as you can... however don't panic if one day you can't get enough... just don't go days without eating or eating very little!