In SK it is reasonable consideration for reasonable time. Meaning it isn't defined by a specific time frame. If you bought the car, put it in your garage and only put 10 km on it, by common law you would have more than 30 days. If you put 4000km on in a week, you could be out of luck. The dealer really has to work with you out of good will, this is where reputation comes in. If the dealer tells you to pound it, you have to go through legal recourse, which may be more hassle than it's worth, and he may in fact be counting on it.