That largely depends on the nature of the contract.
As a practical matter, many local merchants will have "return" policies that permit a buyer to return unused merchandise within a certain time for a full return with no questions asked. Some states have laws giving consumers the right to return merchandise within 7 to 14 days, unless the store predominately posts a "No Return" or "Final Sale" notice.
If the contract involves home repairs, you also have the right to change your mind, typically within 72 hours from signing the contract. The FTC and many states also have "cooling-off" laws involving major purchases, such as new cars.