The syntax rules for Solidity change periodically, there is no predefined time period. As of my last knowledge update in December 2023, the frequency of changes to the syntax rules can vary depending on the development of the language and the Ethereum ecosystem.

To stay up to date with the most recent updates to Solidity syntax rules and other related developments, I recommend subscribing to the "Ethereum Newsletter." This newsletter provides updates on various aspects of the Ethereum ecosystem, including changes to the Solidity programming language and best practices for writing smart contracts.