49 CFR 625.29: Transit asset management plan: horizon period, amendments, and updates.

(a) Horizon period. A TAM plan must cover a horizon period of at least four (4) years.

(b) Amendments. A provider may update its TAM plan at any time during the TAM plan horizon period. A provider should amend its TAM plan whenever there is a significant change to the asset inventory, condition assessments, or investment prioritization that the provider did not reasonably anticipate during the development of the TAM plan.

(c) Updates. A provider must update its entire TAM plan at least once every four (4) years. A provider's TAM plan update should coincide with the planning cycle for the relevant Transportation Improvement Program or Statewide Transportation Improvement Program.