Do I need to leave it overnight or is a few hours okay? I forgot to make it last night and I really want to cover my cakes tonight! What happens if you don't let it sit overnight?

this is probably too late, but for future reference, i think it is fine if it doesn't sit overnight. it will be soft and maybe harder to work with, i prefer to let mine settle, but i know there are many on cc who never let it sit and it seems to work just fine!

you can use it right away. i also have had left over fondant and stored it in an air tight container and placed it in the freezer. i let it thaw and it worked fine for me.