One day of overeating

okay so today was a charity day at my local youth organisation, and one of the competitions was to eat as many things from this "surprise jar" of disgusting delicacies and so on, it helped raise a lot of money, over £200 pounds, purely because I think people were so happy about our misery of having to eat the food, I asked what was the ingredients for it later and I can't remember what they were all exactly as it was like a huge concoction but I did some calculations to see the caloric intake and I've ate around 2500-3000+ calories than what I would usually eat in a day, so I have a few questions:

How long would this weight gain take to show? Would it show up now or tomorrow or monday or vice versa?

I know it's 3500 cals for one lb of fat and so on but I mean jesus I've gained 1/2lbs from overeating by 200-300 calories before and I know it's all "water weight" and so on that's why I want everyone to be blunt with me

How long will the true weight gain show?
How long will it take for me to lose it (I'm currently maintaining and not losing weight as I reached my goal a few weeks ago and have been able to maintain it around that and be happy)
As I am/been maintaining for a few weeks now, will my current diet of maintenance be okay to go back to once the food weights gone or do I still have to be careful about the diet until I'm sure the food weights gone? 

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