Though it depends on the policies of individuals banks, as of 2014, many banks do not cash checks that are more than six months old. Checks like this are referred to as "stale dated."
If six months have passed since a check was originally issued, the check is most likely unserviceable. Checks that have become stale dated may usually be returned to the issuer, who issues a new one. Many employers have procedures in place for reissuing stale-dated pay checks. Checks received from other parties may be more difficult to have reissued. However, there is no reason for the issuing party not to reissue the check, as long as the original has not been cashed.