1. You're right, it varies from person to person. For me, I'm usually in ketosis within about 48 hours of going under 50g of carbs a day. It sometimes depends on how many carbs I've eaten within the days before dropping carb intake though, as it just takes long to burn out those carbs if I was eating a lot of carb heavy meals...then it might take an additional 24 hours, but not much more than that.
2. This kind of depends on how ketoadapted you are. If you do this within the first few weeks of going LC, you're likely to knock yourself out of ketosis with one heavy carb meal. After a few weeks, I can eat carb heavy, and it not affect my ketosis at all.
But, you kind of need to ask yourself why you're even asking question #2. It sounds like you're trying to build in "cheats" instead of looking at this like a lifestyle change. Yes, we all have occasional cheat meals/days, but it's best if they're not a planned, weekly occurrence. That seems like it might be setting you up for failure. I can't overcome carb cravings if I know I'm three days out from a scheduled cheat day. And it makes it more into a cheat day than a cheat meal.
Also, I'm not clear on what "small SAD treat" means. A little vague. What exactly would the cheat be?