I have read that it takes about 2.5 days to use up all the glycogen in muscles and liver before the body goes looking for stored fat to use for fuel. So it might take that long to get back into ketosis.

I have found that when I eat high carb stuff (carbage ) it may take me a week to lose the few pounds that I put on. Some of that is water because each carb gram makes your body hold at least 4 grams of water. Which is why any carbage excursion makes the pounds jump back on.

The more and the longer you can stick to the plan, especially at first, the less likely you are to regain. Some people think that we each have one 'golden shot' at losing all the weight to the goal. And if we mess that one up, the road is going to be a lot longer and a lot slower.

One Golden Shot