I do want to say that I had a Green Tea (with MCT Oil and Butter) before noon. My glucose is currently around 79 (I’m not checking ketones until supper), which is at the upper end of my average for last week. But, its still a good number, and I seem to have recovered from last night’s accidental sugar consumption. And as of around noon my ketones were around 2.6 mmol. Down from what they were before breaking my fast, and only down a little bit from before the “sugar incident”.
Really, I think the length of time it takes to recover is highly individual. You would need to do experiments to find out where the line is for you. Also, remember, it could vary depending on how pure of a sugar (carb) you are eating. Eating more veggies (or milk) could be totally different from eating the same amount of carbs in donut form, for how you react to it.