There are several governing documents pertaining to your retirement plan (e.g., the plan document, adoption agreement and summary plan description, to name a few). Generally, it's a good practice to review these documents on an annual basis. However, circumstances, such as legislative impacts or other changes to your plan, may arise that would require you to review them more frequently.
While not required, many retirement plans will utilize an Investment Policy Statement. This document will define what the available investment options will be for participant directed plans, as well as the investment objectives for the plan. Anytime a new investment option is added or removed from the investment lineup, this document should be amended accordingly.
As a best practice, plan fiduciaries should perform annual plan reviews with their financial professional, third-party administrator and/or legal counsel to ensure that their document is up to date with all enacted legislation and continues to meet the goals and objectives previously determined for the plan.