I've been keto since it was called Atkins Induction in the 1980s or so.

If I cheat, say have one piece of cake, I'll gain a pound that day, and gain for two or three days after that until I get back into ketosis. It takes about a week to get back to my target weight.

One piece of cake, and an entire week of being heavier.

Needless to say, I don't do that often. Perhaps once a year on a very special occasion, and they I'll try to have it follow a high fat/protein meal and make the cake cheesecake or something like that.

Of course YMMV