How many days do you have to return a used car?
None. Car sales in all states are binding contracts the instant they're signed, with a couple of exceptions:
Some (but not all) states require dealers to offer buyers the opportunity to purchase a rescission option. This is generally only required for certain types of sales (for example, only for used cars, and only if they cost less than some particular amount). The dealer is generally allowed to charge for the right itself and a penalty fee if the customer chooses to exercise it.
If you bought the car at a tent sale or in a parking lot and not at the dealership, you MIGHT have the right to cancel the contract within a short period of time (usually 3 days). Usually, the dealer is allowed to charge you a penalty for cancelling in this case also, typically a percentage of the purchase price.
Otherwise ... you can always talk to the dealership. Just because they're not legally required to allow you to return the car doesn't mean they're legally forbidden to do so. Your bargaining position is weak, but if they can get the car back and make some money on the deal, they might just allow it.