How long does it take for high fat and high sugar foods to turn into weight?

I really need to know right now!!! Like, if you over eat by 1,600 calories one day, the next day, how put will i put on/ I am under eight so i have a reaaaaaaly slow mentabolisim. And how long does it take for foods like flapjack, cereal bars and shortbread to digest? Please answer asap!!!!!!!!!!!!!!!!!