If you think of it in terms of using up the chemical sugars and sugar derivatives in you system (muscles, blood & liver), 48 -72 hours, depending on the starting levels AND your level of activity. Obviously, if you hit the cardio really hard you'll burn your sugars faster.

If you're not in ketosis in three-four days, you're probably ingesting hidden carbs.

It could kick in in two days, but don't count on it. Remember, Your Mileage May Vary . . .