It depends greatly on the individual. A few people can enter ketosis quite quickly. Others might need two weeks to accomplish their task. The glycogen in your body has to be burned before you can begin ketosis, so it usually takes a couple of days. You can get into ketosis faster if your diet is lower in carbs. For that reason, some people will do a “fat fast” to kick-start the process, eating 800-1000 calories a day consisting of 90% fat for three days only. This does work, but it works better if you’ve been in ketosis before.