In most severance agreements, you are accepting an amount of money in exchange for a broad release of any possible legal claims against your employer. The claims you would be releasing arising under a wide range of state and federal laws, and there is no one rule about time for consideration that applies to all of them.
If you are over 40 years old, your agreement most likely contains a release of claims under the Age Discrimination in Employment Act (ADEA). This statute has a very specific requirement that you be given 21 days to consider the package, and that you have seven additional days after signing in which you can revoke your acceptance.
Most severance agreements will contain ADEA releases, and therefore offer the 21 day period. For other claims, the requirements are not as specific, but the employer should give you a reasonable time to consider the agreement if the release is going to be enforced.