People used to say 6 montbut I am not sure anyone checks the date. For example the date is ignored on post-dated cheques. You may need to check though that the issuer hasn't cancelled it
Under the cheque act they are valid for six years. Most banks won't clear them after six months as a matter of policy, or will at least cheque with the account holder before clearing them, provided they actually notice. As most cheques are processed automatically they usually don't notice.