If you are soaking the head outside the printer, overnight shouldn't be a problem. You will want to flush the head with a syringe before you put it back in as that will get the debris out of the head before it gets a chance to dry up again. What you are hopping for when you soak a head is for the solvent to wick up into the head to break up the dry ink so it is easier to push out the small nozzles. If you are soaking the head in the machine on the caps, some of my customers have had issues where a siphon occurs and pulls all of the ink out of the system. It rarely happens but it is something you should take into account.
