The response to this sort of thing will vary from person to person.

For me, whenever I had a little naughty for lunch, I noticed it on the scales straight away. I was weighing twice daily at that point, and my weight loss stopped immediately, and didn't resume again for about 3 days. Beyond that though, things carried on again from where they left off.

You may even rebound, or you may notice nothing at all. Either way, the best thing you can do as others have already said, is not to panic, just move on and stick to the plan.