Well for the a good rule of thumb is don't keep a cigar out of a humidor (or similar container with some kind of humidification device) for more than a week. All cigars should be kept in an airtight container, such as a ziploc bag, until moments before smoking.
So if you mean out of the humidor but inside an airtight container, I'd say one week.
If you mean just in the cellophane (which is permeable to air), no more than a few hours.