The purpose of a brute force attack would be to find a private key that would generate the same Public Key as used by your wallet. We call this a 'collision attack'.
As the key we create will have a length of 256bit, that equates to 128 bit entropy. This key length has been accepted the de facto standard in the crypto community. We can cite the bitcoin wiki: "it is more likely that the Earth is destroyed in the next 5 seconds, than that a collision occurs in the next millennium."