Grapes can be left out all day as long as you're planning on eating those grapes while they're out at room temperature.
But you should eat them all within the same day if you do not refrigerate them.
You should not leave the grapes out overnight because they can go bad and start growing bacteria and mold on them.
It's recommended that you don't leave grapes out longer than 2 hours at room temperature to keep them fresh and avoid bacteria forming on them.
If the grapes are left out overnight they might still be safe to eat but inspect them for mold growth and if they smell bad or taste bad then throw them out.