This is highly individual, but most people find that they have as good or better energy levels within the first two or three weeks, while some such as myself feel better right away. There are a few individuals who may find that it takes longer than 3 weeks to gain their energy levels back, but always look for other things that may not be optimal if you find yourself in this situation.
Technically speaking the entire process of becoming adapted to ketones as fuel can take a number of months, but in crossover studies in athletes by professor Tim Noakes of South Africa, he found that athletes would functionally have adapted by 3 weeks and be able to perform at the same level they did previously, and now could continue that performance much longer as they are running on their fat fuels instead of limited glycogen stores.
(Carnivore decathlete Ryan Talbot)
Some wonder if it wouldn't be faster to replenish their glycogen stores by eating carbs and sugar during the workout, but thankfully we have studies now showing that when keto adapted, the zero carb athlete replaces their blood sugar and glycogen levels faster than a carb fuelled athlete who is drinking down sugar water. So this is optimal not only for weight loss but for athletics as well.
(Sean Sakinofsky, carnivore endurance cyclist)
The great part about this is that the benefits don't just stop there as far as energy and function goes. As the months go on your body will replace old, worn out cells through a process called autophagy, but not only that they will replace organelles in the cells, sort of like replacing worn out parts in a car. Mitochondria are the powerhouses of the cells, but more importantly they run the cells and their actions by physically moving around and turning on and off different organelles, receptors, and even genes. Through a process called mitophagy the cells replace old, worn out mitochondria and then stimulate further production of the mitochondria so that within a few months it has been shown in studies that you will have four times the number of mitochondria, and they will be four times as effective, giving a massive physiological boost to your cells and organs.
Dr. Anthony Chaffee