You are using a cipher with a variable key length and need to decide which key length to use.
Strike a balance between long-term security needs and speed requirements. The weakest commonly used key length we would recommend in practice would be Triple-DES keys (112 effective bits). For almost all other algorithms worth considering, it is easy to use 128-bit keys, and you should do so. Some would even recommend using a key size that’s twice as big as the effective strength you’d like (but this is unnecessary if you properly use a nonce when you encrypt; see Section 5.3.3).
Some ciphers offer configurable key lengths. For example, AES allows 128-bit, 192-bit, or 256-bit keys, whereas RC4 allows for many different sizes, but 40 bits and 128 bits are the common configurations. The ease with which an attacker can perform a brute-force attack (trying out every possible key) is based not only on key length, but also on the financial resources of the attacker. 56-bit keys are trivial for a well-funded government to break, and even a person with access to a reasonable array of modern desktop hardware can break 56-bit keys fairly quickly. Therefore, the lifetime of 56-bit keys is unreasonable for any security needs. Unfortunately, there are still many locations where 40-bit keys or 56-bit keys are used, because weak encryption used to be the maximum level of encryption that could be exported from the United States.