Does washing kimchi just before eating it reduce its sodium content much, or do I have to put it in water overnight or several days?
Answer depends on how the kim chi is made.
I make it using a variant on this recipe. The protocol there is to treat the leaves with a salty brine for 4 hours, then rinse them extensively.
No further salt is added in the recipe, so any salt in the final product will have osmosed into the leaves. Most of that salt won't come back out except with a prolonged soak*.
I've seen other kim chi recipes that do involve adding back salt after the initial brining and rinse. The sodium content of those could be reduced with a quick rinse, but you'll likely lose flavor too.