hi jan jan
yah, usually within few hours the ants will disappear. don't need to finish the whole box at once, you just need to pour a small amount along the line where the ants are going to and fro... they will pick the bits back to their nest.
the remaining unused amount, i usually seal it up with tape and keep for next time.
i think should be okay to keep in the kitchen, but not with food items of course.