A ketogenic diet is a low carb, high fat diet that turns your body into a fat-burning machine. When insulin - the fat storing hormone - levels drop, you will feel the difference of an optimal body. Keto has many weight loss, health and performance benefits for type 2 diabetes, epilepsy, PCOS, metabolic syndrome, blood pressure, cholesterol, mental focus and numerous autoimmune diseases.
In short, whatever you want, so long as it doesn't conflict with the core prinicples of the diet. Contrary to what you may think, keto isn't some crazy resritctive diet. You can be vegetarian, vegan, or eat bacon for every meal. Diverse meal plans, and balanced diets are not hard to come by. The general rule and most limiting factor is to limit yourself to 20 or less carbs a day. HOWEVER! All carbs are not created equal. Simple carbs found in foods such as fruits, and nuts are preferable to complex carbs found in foods like bread, and pasta. Additionally, fiber (which is a carbohydrate), does not count against your daily carb-intake, as it cannot be digested by your body, and does not cause an insulin response.
Although keto isn't the most resritctive diet in the world, many of your favorite foods will be off limit. Pasta, potatoes, bread, rice, chocolate, donuts, cake, and pizza are sadly all off the menu. In addition to these Items you should be careful with fruit consumption. Fruit is packed with sugars that when eaten, cause an insulin response (something you want to avoid on Keto);
Water...water....and more water.Well maybe not all the time, but most beverages we drink are filled with sugars and carbs. Diet sodas are technically okay, but even when not on keto, they should probably be avoided. Coffee with no sugar, and very limited amounts of milk and cream may be fine. Likewise with tea.
You can do a keto diet for as long as you feel comfortable. Some people choose a keto diet as a means to lost weight, so in that case it can be looked at as a short-term diet. However, keto is not only a weight loss diet, and in other cases people feel physically and mentally better on keto, than they did when they were off keto. The most important thing you can do is to listen to your body, and adjust you diet accordingly.
If you aren't in it for the weight-loss benefits than that is completely fine. There have been numerous studies that suggest that being in ketosis can boost cognitive funciton, stabilize moods, and even benefit those with neurological disorders. Sleep disorders, memory, and new learning, are just a few things that can be improved by being in ketosis. Here's a few links if you don't believe us: