Motivation for frequent key updates I am wondering if there is any reason to regularly update cryptographic keys, that are used for authentication purposes in a real-time system. 

A key should be updated if it is weak, broken, compromised, or leaked. Another valid reason would be a weak authentication scheme. For instance, CBS-MAC is prone to message forgery in case messages of different length are authenticated using the same key. AES-GCM with the same IV is careless if the same key is used. 

As Swashbuckler pointed out, block ciphers leak information and therefore, encryption keys should be updated. With regard to authentication, I do not see such a reason, because there is no confidential data and even if the adversary observed a collision, he could not learn the key (in most cases). 

Key updates, hence, do not give any benefit from a pure security point of view if a strong cryptosystem is deployed. Do I oversee something?
All block ciphers leak information.  No matter the mode used there is a limit on the amount of data that should be encrypted with a single key, irrespective of the number of different IVs used, in order to not leak too much information that an attacker might be able to determine some of the encrypted data.  For AES, it seems that the limit seems to be around 64 GB no matter the mode.

See the book Cryptography Engineering: Design Principles and Practical Applications and the paper The Missing Difference Problem, and its
Applications to Counter Mode Encryption.