Few hours maybe. But why do you want to know?
Whether you're in ketosis or not has no effect on the diet working well. It helps reduce feelings of hunger (though the jury's out on why).
There's a chance that if you eat a significant amount of carbs in one go, say more than 100g worth, you'll refill your glycogen stores and therefore retain water until they're emptied again. So a splurge on carbs might lead to several pounds increase on the scales - but this would only be a temporary 2-3 day effect - until your body's had a chance to burn off your glycogen stores again.
Calories, on the other hand, do matter...