I'm not a nutritionist but 1100 calories a day is very, very low for most adults. As a comparison, the Minnesota starvation study fed participants a little over 1500 calories a day.
This is better known as the Protein-Sparing Modified Fast (PSMF). Still an intense diet — after all it's an intelligent way to crash diet.
Long-term... not such a good idea unless you are obese.
That number also varies between people. 1500 I would maintain my weight or gain slightly.