Mostly it will depend on your existing glycogen stores. Both in the muscles and liver. If you didn't eat any carbs the evening before, woke up in the morning to 45-60min. of fasted cardio, and ate nothing but flax oil (and H2O) for 6 meals I think you'd surely be in ketosis by the end of the day.
Oh yeah, you'll probably feel like ass. But I suppose that goes without saying.