I've done it a day before and it's been fine. Much longer than that and you'll start losing smoke character unless you seal it up tight,
One recommendation, consider cold smoking. I have the Pro-Q cold smoke generator and it works awesome. A guy I work with made something similar with one of those mesh splatter screens that go over a fry pan. You bend it into an "M" shape and fill it with sawdust (available at some BBQ shops, Cabelas, etc) in the v-shaped portion of the "M". Light one end and it should slowly smolder for at least a few hours. Since you get smoke without heat, you don't have to.worry about darkening your grain.