How long does it has to be before buyer can change his mind about sell?

The duration a buyer has to change their mind about a purchase varies based on the laws of the jurisdiction and the specific terms of the sale. In many cases, a buyer may have a "cooling-off" period of a few days to a couple of weeks, especially for certain types of purchases like door-to-door sales or contracts. However, for most retail purchases, once the sale is completed, the buyer typically cannot return the item unless the seller has a return policy allowing for it. Always check the specific return or cancellation policy associated with the purchase.