my dealer gave me the courtesy of 48 hours, but realistically some will give you the car so long as you sign a promissory note stating you'll come up with the cash within so many days.. that's what mine did, gave me a week to secure financing..
edit: my sales guy basically told me that people were asking about it, they'd had cash offers on mine and that if it hung around long enough for the sales manager to notice a lack of movement, i might find it missing with my deposit refunded.. not exactly honorable, but they've got a business to run and they're hemorrhaging at the moment..