40 CFR § 1502.5 - Timing.
An agency should commence preparation of an environmental impact statement as close as practicable to the time the agency is developing or receives a proposal so that preparation can be completed in time for the final statement to be included in any recommendation or report on the proposal. The statement shall be prepared early enough so that it can serve as an important practical contribution to the decision-making process and will not be used to rationalize or justify decisions already made (§§ 1501.2 of this chapter and 1502.2). For instance:
(a) For projects directly undertaken by Federal agencies, the agency shall prepare the environmental impact statement at the feasibility analysis (go/no-go) stage and may supplement it at a later stage, if necessary.
(b) For applications to the agency requiring an environmental impact statement, the agency shall commence the statement as soon as practicable after receiving the application. Federal agencies should work with potential applicants and applicable State, Tribal, and local agencies and governments prior to receipt of the application.
(c) For adjudication, the final environmental impact statement shall normally precede the final staff recommendation and that portion of the public hearing related to the impact study. In appropriate circumstances, the statement may follow preliminary hearings designed to gather information for use in the statements.
(d) For informal rulemaking, the draft environmental impact statement shall normally accompany the proposed rule.