How long does human body have to go without food/sustenance before it starts burning its own fat for energy?

I've been Googling this, and have found profoundly conflicting information. I know that body size and rate of metabolism etc are factors in this, but there has to be a somewhat accurate estimation of how long it takes. I've seen sources say after a day (24 hours) of no food the body rapidly starts to eat away at its own fat and then muscles etc while other sources say it'll take 4-5 days and so on.