You can expect to reach a state of ketosis in a matter of days if you follow a traditional keto diet. Fats convert directly into ketones which is why it's important the majority of your diet comes from healthy fats. If you get the majority of your calories from carbs or protein, this will wreck your ketone levels.  Again, if you want to reach ketosis faster, in a very short time, you'll need to expedite this process and have an overall different approach. Fasting and exercising are the two ways to do this. 

In extension it is also great to have solid meal plan. A solid meal plan will ensure that the you obtain the correct foods at the right time during each day of your keto journey. It is recommended that your meal plan is specific for you and designed to track your daily carb intake. To ensure you get into ketosis in the shortest possible time, it is imperative that you keep your carbs between 20- 30 grams daily. On the other hand your ketogenic resources should be anywhere between 70 - 80% of your daily calorie intake.  

In order to avoid "keto flu" symptoms it's important to drink a lot more water than you would normally. When glucose stores are depleted over a short period of time, you not only lose a lot of water, but you also become depleted in water-soluble minerals like sodium, potassium and magnesium. If you have deficiencies in any one of these elements you will definitely feel it. Therefore, mineral water or a mineral supplement is best to consume because it replaces what is lost during the fasting process. 

It's possible to get into ketosis in a very short time (24 hours minimum) by taking strict actions such as fasting and high intensity exercising. It's important to take mineral supplements and drinking a significant amount of water daily in order to prevent negative side effects.