what is a public key's life cycle?

Although the key life cycle seems to be an important factor in cryptosystems' security, I couldn't find a solid refrence on the topic. After hours of web search, a discreption on RSA Lab's website is satisfactory.
It says: "Research in cryptanalysis can lead to possible attacks against either the key or the algorithm. For example, recommended RSA key lengths are increased every few years to ensure that the improved factoring algorithms do not compromise the security of messages encrypted with RSA. The recommended key length depends on the expected lifetime of the key. Temporary keys, which are valid for a day or less, may be as short as 512 bits. Keys used to sign long-term contracts for example, should be longer, say, 1024 bits or more. "