I think it depends how much carb you ate on your big day out. I am in ketosis now, day 4, after i had a binge which involved LOTS of carby foods (several bags of crisps, bread, etc). You will get back into ketosis much quicker if you didn't overdo the carbs too much, maybe within a day or 2 days, otherwise it's similar to starting from scratch (so up to a week).