Short answer: Not enough.
The AES algorithm is defined in the FIPS standard with keylenght of 128, 192 or 256 bits. So you cannot use directly a 56-bit key. One needs to have a key with the proper length to use the AES encryption algorithm.
Data will be protected using AES-256 encryption with a 56-bit effective key length
probably means that the key used for the encryption has been generated with an entropy on only 56 bits.
One can imagine multiple ways to do such operation: hashing with a function a random buffer of 56 bits or take random 56 bits and pad them to 256 with zeroes (or with any fixed, public, know value) or ...
In other words the quoted sentence means that a brute force would take only $2^{56}$ operation despite the keylenght is 256.
As today AES-256 is considered secure (the best known attack has complexity of $2^{254}$) one can consider that the effective strength of the encryption would be 56 bits.
From the "Public" column at the Keylength.com website one can remark that the required security level for symmetric encryption is (at present moment) at least 79 bits. Therefore an AES-256 with a 56-bit effective key length should not be considered as secure.
Reading this paper may result useful to understand how long a 56-bit key would resist to a bruteforce attack.