1200 calories is absolutely fine even long term as long as it's the right kind of calories. If you're eating 500cals a day of empty carbs like bread or pasta etc, then obviously you're going to be hungry because you're not feeding your bodies needs and you're going to spike your blood sugar with carbs.
However, if 10% of those calories only come from carbs, and highly nutrient dense carbs at that, then 20% from good quality protein and 70% from fat, your blood sugar will stay really stable, you wont trigger an insulin response and you can maintain that state of nutritional ketosis indefinitely. The sensation of hunger is our bodies way of telling us our blood sugar levels are dropping, so if you stop that from happening, you're golden :)
It sounds like a really odd way to eat i Know, I was sooo skeptical when I started out but actually, the food is frustratingly normal
I'm sure people think LCHF'ers just eat sticks of butter...
Daily food for me looks like this:
Breakfast:
Coffee with 50mls double cream in it
Lunch:
Usually a plate of salad leaves with either:
Smoked salmon gravadlax and avocado
Chicken, parmasan, tomatoes and avocado with ceasar dressing
Bacon, avocado, tomato and full fat mayo
Tuna, full fat mayo and avocado
Egg mayo and avocado
Or, if im in a hurry and need a ready meal, tesco finest cauliflower cheese 
Dinners:
Slow cooked lamb shank, asparagus cooked in butter and gravy made from the lamb stock
Steak, creamy stilton sauce and roasted broccoli
Salmon with spinach and lemon butter sauce
Prawns, stir fry veggies and sauce made from a big scoop of peanut butter
Lamb or beef Burgers but instead of a bun, use 2 flat mushrooms with garlic mayo
If it's a takeaway night, kebab (chicken or donar) with salad and mayo without the pita bread
Belly pork roasted with spinach/ kale/ asparagus/ cauli etc
Lamb shank shredded through cauliflower rice with moroccan spices
with plenty of butter in it
I'm a massive foodie so it suits me down to the ground :) Because the meals have a high fat content, they are all super fast to cook or done in the slow cooker. I can have a meal for 2 on the table in 20 minutes most nights :)