From the wikipedia page (http://en.wikipedia.org/wiki/Ketosis)
When glycogen stores are not available in the cells, fat (triacylglycerol) is cleaved to provide 3 fatty acid chains and 1 glycerol molecule in a process called lipolysis. Most of the body is able to use fatty acids as an alternative source of energy in a process called beta-oxidation. One of the products of beta-oxidation is acetyl-CoA, which can be further used in the Krebs cycle. During prolonged fasting or starvation, acetyl-CoA in the liver is used to produce ketone bodies instead, leading to a state of ketosis.
During starvation or a long physical training session, the body starts using fatty acids instead of glucose. The brain cannot use long-chain fatty acids for energy because they are completely albumin-bound and cannot cross the blood-brain barrier. Not all medium-chain fatty acids are bound to albumin. The unbound medium-chain fatty acids are soluble in the blood and can cross the blood-brain barrier.[2] The ketone bodies produced in the liver can also cross the blood-brain barrier. In the brain, these ketone bodies are then incorporated into acetyl-CoA and used in the citric acid cycle.
I interpret that as, you burn sugar first, then fat second, after sufficient time buring fat you start burning the actyl-CoA as ketones. How long that "sufficient time" is probably varies greatly from individual to individual, so the only want to know for sure is to test it. I'm pretty sure that if I skip lunch, by the time I get home from work I'm in ketosis.
