[kjksf]

Your body starts producing ketons when you stop eating. Typically it starts 12 to 24 hrs.

It'll start faster if you don't eat carbs / sugar and instead use protein because the body start producing ketons when it runs out of glycogen stores. Sugar and carbs add glucose to your blood. The less of it you have, the faster the body will start producing ketons.

Exercise also helps because it uses glucose faster. Of course assuming you'll not just start eating carbs / sugar after exercise. That includes all drinks that include sugar.

But really you'll get most benefit of exercise just from a walk after meal.

Why after meal? Because that's when glucose spikes and a walk uses glucose faster.

[dcow]

You have it backwards. Your liver can convert protein into glucose in a process called gluconeogenesis https://en.wikipedia.org/wiki/Gluconeogenesis

[ericmcer]

Don’t you produce ketones any time you can’t get enough energy from carbohydrates?

[EasyMark]

Your body usually stores 1-2 days worth of “glucose” energy in the form of glycogen in muscle tissue and the liver