Lets say you eat something very salty. Then you gain some water weight, because of it. How many days will it take for this water weight to wear off, after you've stopped eating that thing?

It will vary from person to person depending on body composition, amount of water you drink, etc. Probably a few days.

I guess it depends but from looking at my daily weigh ins I think 2-3 days. It can go on for longer though, I guess it's different per person.

for me, it takes anywhere from 2 days to a week before my body settles down from the sodium. :/