What is the test for establishing a substantial relationship?

"If a substantial relationship is established, the discussion should ordinarily end. The rights and interests of the former client will prevail. Conflict will be presumed; disqualification will be ordered." (River West, Inc. v. Nickel (1987) 188 Cal.App.3d 1297, 1308; see also, e.g., City and County of San Francisco v. Cobra Solutions, Inc., supra, 38 Cal.4th at p. 847.) "However, it is not in the interests of justice to make the `substantial relationship' rule so unyielding as to permit the former client to inexcusably postpone objections without penalty." (River West, Inc. v. Nickel, at p. 1309.) "The trial court must have discretion to find laches forecloses the former client's claim of conflict." (Ibid.) But " `mere delay' in making a disqualification motion is not dispositive. The delay must be extreme in terms of time and consequence." (Id. at p. 1311.)